feat(terminal): take the transcript gutter off a copy, at the width the CLI declares

Copying a paragraph out of a Claude Code or Codex pane puts that pane's own
two-column transcript gutter on the clipboard, so every pasted line arrives
indented. #451 shipped the trailing half of the copy clean and left the leading
half out, because deriving the width from the selection fires on 73% of ordinary
indented text and cannot tell a margin from content.

The width is DECLARED rather than derived. `capabilities.transcriptGutter` on
the CLI registry is a bounded integer; claude and codex each declare 2, measured
on live panes, and no other stock entry declares any, so a CLI whose transcript
layout nobody has measured is never touched. The server publishes the map as
`window.__codemanTranscriptGutter`, built by filtering `enabledClis()` on the
capability rather than by listing ids, and `_activeCliGutterColumns()` looks the
active session's mode up in it. The copy path reads no terminal buffer at all.

The declared width is a CEILING, not the answer: `clean()` strips the lesser of
it and the run every selected line shares. A block can therefore only shift as a
unit, the structure inside a selection survives by construction, and a selection
reaching column 0 loses nothing. That is what keeps a `git log` body at its own
four-space indent inside an agent's two-column gutter.

Codex was measured separately, because it renders nothing like Claude: it draws
boxes narrower than the pane and pushes its transcript into ordinary scrollback.
On a live 0.154.0 answer its `•`/`›`/`⚠` markers sit in the gutter, prose
continuations sit at 2, and a nested YAML block the model wrote rendered at
2/4/6/8 for its own 0/2/4/6. Replayed at 100, 120, 160, 198, 235 and 282 columns
its indents were 0, 2, 4, 6 and 8 at every one, never 1. Copying that YAML out
of a live Codex pane now yields 0/2/4/6: gutter gone, nesting intact.

Two derived versions were built and measured first, and both are recorded in the
code because both looked correct:

- Painted trailing padding — a full-screen TUI writes real spaces across the
  unused part of a row, a shell leaves them never-written for xterm to trim —
  has no false positives and never over-stripped. It is also a function of pane
  WIDTH: the padding exists only while a rendered line stops short of the CLI's
  own layout width, and Claude's prose wraps to fill it. Dragging the same two
  prose rows of one live transcript at five window sizes, the share of padded
  rows ran 44%, 6%, 6%, 7% and 87% at 123, 160, 198, 235 and 298 columns, so the
  strip silently did nothing at every ordinary size while a corpus captured
  entirely at 282 columns said it worked.
- Taking the narrowest indent on the rows around the selection fires at every
  width and over-strips about 1% of selections, because a file listing inside
  the transcript can be the narrowest thing on screen.

Measured over 1,392,281 selections — every 1, 2, 3, 5, 10 and 20-row window of
real Claude screens replayed from live PTY streams at 100, 120, 160, 198, 235
and 282 columns — the declared width over-strips none, breaks no relative indent
and alters no text, and serves 100% of the selections whose own indent covers
the gutter. Verified end to end in a browser with a real mouse drag and a real
Ctrl+C: Claude and Codex panes paste flush at 123, 198 and 298 columns, a shell
pane is untouched at every one.

The strip sits behind `copyStripMargin` (App Settings, Selection & clipboard),
per-device and default ON: a display key, absent from the .strict()
SettingsUpdateSchema, read as `!== false` because the desktop branch of
getDefaultSettings() returns {}. The toggle is checked before the map.

Two review findings from #451, handled:

- The mid-row flag governs ONE line now. `range.start.x > 0` excludes only the
  first selected line, the one whose margin the mousedown genuinely cut off, so
  the same three rows no longer produce three different clipboard results.
- The reversed-drag finding does not reproduce on the pinned xterm.
  `getSelectionPosition()` reads `_selectionService.selectionStart`, whose
  getter returns `SelectionModel.finalSelectionStart`, and that swaps the pair
  when `areSelectionValuesReversed()` says so. A real upward mouse drag through
  chromium against xterm 6.0 reports the same range as the downward drag.
  `_normalisedSelectionRange()` keeps the ordering as a guard, because the model
  one layer down exposes the unnormalised fields under the same two names.

Tests: test/terminal-copy-clean.test.ts (64, up from 31), plus the injected
script stripped in test/server-index-title.test.ts. Every guard is pinned:
removing any one of seven reds at least one test, including declaring the wrong
gutter width. Full suite green, 7,861 passed, 0 failed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

test/server-index-title.test.ts

@@ -96,9 +96,9 @@ describe('WebServer index.html <title> templating (#82)', () => {
 
   it('only substitutes the <title> tag — the rest of the template is identical (modulo asset cache-busting)', async () => {
     // renderIndexHtml also appends ?v=<mtime> cache-bust params to same-origin
-    // .js/.css refs, and injects the CLI-availability flags plus the custom-model
-    // Run-menu picker's CLI list before </head>; strip all so the title remains
-    // the only other change.
+    // .js/.css refs, and injects the CLI-availability flags, the custom-model
+    // Run-menu picker's CLI list and the transcript-gutter widths before </head>;
+    // strip all so the title remains the only other change.
     //
     // The flag strips are what keep this test environment-independent. The
     // CLI-availability one used to pass here by luck: that script was injected
@@ -109,7 +109,10 @@ describe('WebServer index.html <title> templating (#82)', () => {
     const html = (await render('laptop'))
       .replace(/(\.(?:js|css))\?v=[^"]*/g, '$1')
       .replace(/<script>window\.__codemanCliAvailable=\{.*?\};<\/script>\n/, '')
-      .replace(/<script>window\.__codemanCustomModelClis=\[.*?\];<\/script>\n/, '');
+      .replace(/<script>window\.__codemanCustomModelClis=\[.*?\];<\/script>\n/, '')
+      // Injected unconditionally as an object keyed by run mode, empty when no
+      // enabled CLI declares a gutter, so it needs stripping on every machine.
+      .replace(/<script>window\.__codemanTranscriptGutter=\{.*?\};<\/script>\n/, '');
     const beforeTitle = rawTemplate.split('<title>Codeman</title>')[0];
     const afterTitle = rawTemplate.split('<title>Codeman</title>')[1];
     expect(html.startsWith(beforeTitle)).toBe(true);