fix(session): let each CLI declare how its own pane shows work

A Codex session reported `isWorking: false` for its entire life, including
mid-turn. Codeman has four paths that mark a session working, and all four were
inert for Codex:

- The spinner fast path tests eight braille frames, and Codex animates none.
- The activity-streak fallback was wrapped in `!isExternalCliMode(mode)`.
- The pane probe inside `_confirmIdle` would have matched, since Codex prints
  `esc to interrupt`, but arming it required the literal glyph `❯` and Codex
  draws `›` on its composer row.
- The text detector sat inside `_processExpensiveParsers`, whose first statement
  returns early for an external CLI.

Add an optional `workDetect: { promptGlyph, workingLine }` to CliCapabilities,
so the two strings that differ per CLI are registry data rather than constants
in the detector. Claude declares its existing pair and behaves as before. Codex
declares `›` and `esc to interrupt`. The text detector moves above the
external-CLI early return, guarded on the descriptor so a CLI without one still
skips the ANSI strip that the early return used to save it.

A CLI that declares no descriptor falls back to Claude's pair, and the
activity-streak gate now reads "has a descriptor, or is not external", so the
plain shell mode keeps the behaviour it had.

Rewrite the test that asserted the old premise in its own comment, so it makes
the same guarantee for a genuinely uncharacterised CLI, and add Codex coverage
built from verbatim pane captures on Codex CLI 0.152.1.

src/config/cli-registry/schema.ts

@@ -274,6 +274,26 @@ const capabilitiesSchema = z
     effort: z.boolean(),
     agentSkillInjection: z.boolean(),
     statusLineTelemetry: z.boolean(),
+    workDetect: z
+      .object({
+        promptGlyph: z.string().min(1).max(8),
+        // Compiled per session, so a broken pattern must fail at LOAD time rather than
+        // throw inside the PTY data handler.
+        workingLine: z
+          .string()
+          .min(1)
+          .max(400)
+          .refine((src) => {
+            try {
+              new RegExp(src);
+              return true;
+            } catch {
+              return false;
+            }
+          }, 'workingLine must be a valid regular expression'),
+      })
+      .strict()
+      .optional(),
     model: z
       .object({ source: z.enum(['flag', 'claude-settings-file', 'none']), param: z.string().optional() })
       .strict(),