feat(sessions): land auto-naming opt-in, in the prefix form, from the first user prompt only

Finishes #376. The contributed keystroke tracker sat on the raw byte stream
and named tabs wrong five ways (every prompt, every write path, a bare Esc
eating the next prompt's first character, pasted newlines as Enter, any CSI
clearing the draft) and replaced the whole name, which dropped the case from
the tab and reset the w<n> counter. This lands the feature with each of those
closed:

- First prompt means the first: applyAutoName() flips a placeholder to
  `auto` whether or not the string changed. nameSource is now the tri-state
  placeholder | auto | manual; the name setter is the only manual path.
- Only user-originated input counts: write()/writeViaMux() take
  SessionWriteOptions.fromUser, set by the browser WS path and POST /input
  only, so Ralph, respawn, cron, approvals and the trust-dialog keys can
  never name a tab. A startMode 'shell' CLI never feeds the tracker (a
  capability, not an id check); the send-key route feeds trackUserInput()
  because its line feed bypasses the session.
- Prefix form `w3-case: title`: parseSessionPrefix() already renders it as
  the title with the prefix in the tooltip and the next-session counter
  still matches it. Composed within MAX_SESSION_NAME_LENGTH.
- Tracker rules per key: bare Esc resolves at chunk end; mouse/focus
  reports, Tab, cursor keys, Shift+Tab are no-ops; Up/Down and Ctrl+P/N/R
  taint the draft so Enter submits nothing rather than a fragment;
  bracketed-paste newlines and Ctrl+J / Shift+Enter join with one space;
  the draft keeps its head past 8192 code points; an escape past 64 bytes
  is abandoned.
- Title: slash commands by shape (a path is a prompt), `!` escapes
  refused, first sentence only past 8 code points ("e.g." is not a title),
  72 code points on a word boundary.
- Synced `autoNameSessions` setting, default OFF (the prompt reaches
  mux-sessions.json, session:updated and /api/search), App Settings ->
  Appearance -> Tabs, read fresh per prompt after the eligibility check.

Tests: test/session-auto-name.test.ts (tracker, title, composition,
ownership, emit gating), the wiring test (once, prefix, setting off,
manual protected), test/routes/session-name-routes.test.ts (PUT /name
flips to manual and persists). Verified live on an isolated instance: API
and browser-typed prompts name the tab, a second prompt does not, shells
and renamed tabs are untouched, nameSource survives a restart.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/web/routes/session-routes.ts

@@ -1081,7 +1081,6 @@ export function registerSessionRoutes(
       workingDir,
       mode,
       name: body.name || '',
-      nameSource: body.name ? undefined : 'auto',
       mux: ctx.mux,
       useMux: true,
       niceConfig: globalNice,
@@ -1596,6 +1595,9 @@ export function registerSessionRoutes(
 
     // Write input to PTY. Direct write is synchronous; writeViaMux
     // (tmux send-keys) is fire-and-forget to avoid blocking the HTTP response.
+    // Every write here is `fromUser`: this route carries a person's prompt, or an
+    // agent's on their behalf, so it may name the tab (Ralph, respawn, cron and
+    // approvals write through the session directly and never say so).
     //
     // Because the response has already been sent by then, a failure there is the
     // one case the caller can never learn about — so the dedup bookkeeping is
@@ -1618,32 +1620,32 @@ export function registerSessionRoutes(
     } else if (useMux && waitPromise) {
       // The response is already staying open for the wait, so the tmux write can be
       // awaited here. This is the ONE path where a writeViaMux failure is observable.
-      const ok = await session.writeViaMux(inputStr).catch(() => false);
+      const ok = await session.writeViaMux(inputStr, { fromUser: true }).catch(() => false);
       if (ok) {
         delivered = true;
       } else {
         console.warn(`[Server] writeViaMux failed for session ${id}, falling back to direct write`);
-        delivered = session.write(inputStr);
+        delivered = session.write(inputStr, { fromUser: true });
         if (!delivered) undoOnFailure();
       }
     } else if (useMux) {
       // Fire-and-forget: don't block the HTTP response on a tmux child process.
       // Fallback to a direct write on failure. Unchanged from before send-and-wait.
       session
-        .writeViaMux(inputStr)
+        .writeViaMux(inputStr, { fromUser: true })
         .then((ok) => {
           if (ok) return;
           console.warn(`[Server] writeViaMux failed for session ${id}, falling back to direct write`);
-          if (!session.write(inputStr)) undoOnFailure();
+          if (!session.write(inputStr, { fromUser: true })) undoOnFailure();
         })
         .catch(() => {
-          if (!session.write(inputStr)) undoOnFailure();
+          if (!session.write(inputStr, { fromUser: true })) undoOnFailure();
         });
     } else {
       // Same rollback. NOT an error response, deliberately: a session can
       // legitimately have no PTY yet (created but not started), and callers have
       // always been able to write to one without a 4xx.
-      delivered = session.write(inputStr);
+      delivered = session.write(inputStr, { fromUser: true });
       if (!delivered && tagged) {
         session.forgetInputSeq(clientId as string, seq as number);
       }
@@ -1892,6 +1894,9 @@ export function registerSessionRoutes(
       console.error('[Server] send-key failed:', err);
       return createErrorResponse(ApiErrorCode.INTERNAL_ERROR, 'tmux send-keys failed');
     }
+    // The bytes bypassed the session's write path, so tell the auto-name
+    // tracker about them or the two lines of a prompt join with no separator.
+    session.trackUserInput(hex.map((byte) => String.fromCharCode(parseInt(byte, 16))).join(''));
     return {};
   });
 
@@ -3485,7 +3490,6 @@ export function registerSessionRoutes(
     const session = new Session({
       workingDir: resolvedCasePath,
       name: sessionName ? sessionName.slice(0, MAX_SESSION_NAME_LENGTH) : '',
-      nameSource: sessionName ? undefined : 'auto',
       mux: ctx.mux,
       useMux: true,
       mode: mode,

src/web/routes/ws-routes.ts

@@ -185,7 +185,8 @@ export function registerWsRoutes(app: FastifyInstance, ctx: SessionPort, getHost
               // Typed input from a claim-holding desktop keeps the claim "hot"
               // and re-asserts the desktop layout after a mobile override.
               if (holdsDesktopClaim) session.noteDesktopActivity();
-              delivered = session.write(msg.d);
+              // Browser keystrokes are the user's own, so they may name the tab.
+              delivered = session.write(msg.d, { fromUser: true });
               // A session whose PTY is gone swallows the write. ACKing anyway told
               // the client to drop the frame from its durable queue and left the seq
               // burnt, so the retry that reliable delivery exists for was rejected as