feat(web): toggle thread pin from the keyboard

[rynfar]

Pinning a thread required reaching for its context menu. Adds mod+shift+p to
pin or unpin the thread you have open.

The shortcut is gated on the environment's threadPinning capability, ignored
while the terminal has focus, and reports failures through the usual thread
toast. Pylon already had the whole substrate — threadPinning: true in
ServerEnvironment, and pinThread/unpinThread in client-runtime — so this
only wires the keyboard path to it.

Adopted from T3 Code 4c51b4c9b6a85d96a22e0df41d5cfd2d8fc9901d, cherry-picked
clean with no adaptation.

Both directions are covered, so this is not a one-way door: the same shortcut
unpins. Because the command is added to THREAD_KEYBINDING_COMMANDS in
contracts, it also shows up in Settings → Keybindings for rebinding without any
extra label wiring — checked rather than assumed.

Verification

• vp test run apps/server/src/keybindings.test.ts — 22 passed
• Mutation probe: removing the mod+shift+p default fails the test
  (expected undefined to equal 'mod+shift+p')
• vp run -F @t3tools/web typecheck, -F @t3tools/contracts typecheck,
  -F @t3tools/shared typecheck — all clean
• Docs updated by the same commit: docs/user/keybindings.md and
  docs/user/thread-sidebar.md, both already in Pylon's voice with no T3
  references

Keyboard-only change, so no screenshots.

Claude Opus 5 via Claude Code.

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[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.3 KiB	13.3 KiB	+58 B (+0.4%)	15.1 KiB	✅
Codex	Thread snapshot wire	6.9 KiB	6.9 KiB	+1 B (+0.0%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.4 KiB	6.4 KiB	+57 B (+0.9%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	55.5 KiB	55.6 KiB	+44 B (+0.1%)	66.4 KiB	✅
Codex	Live turn messages	9	10	+1 (+11.1%)	21	✅
Claude	Total thread wire	13.3 KiB	13.3 KiB	−5 B (−0.0%)	15.1 KiB	✅
Claude	Thread snapshot wire	6.9 KiB	6.9 KiB	+7 B (+0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.4 KiB	6.4 KiB	−12 B (−0.2%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	56.4 KiB	56.4 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	10	10	0 (0.0%)	21	✅

Baseline: 2b7a9b8 · PR result: 51ac00b · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 109.4 KiB
• Claude decoded thread snapshot: 110.1 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.

[rynfar]

Independent review pass. One low-confidence finding, nothing fixed.

mod+shift+p is a Firefox-reserved accelerator (New Private Window) that
preventDefault cannot reclaim, so the shortcut is dead in Firefox. Noted as a
tradeoff rather than a defect: mod+shift+n already ships with the same class of
conflict against Chrome's incognito window, and the command is rebindable in
Settings → Keybindings because it goes through THREAD_KEYBINDING_COMMANDS.

The review confirmed the things I would have wanted checked:
pinThread/unpinThread signatures match the sidebar and context-menu callers,
both re-check the threadPinning capability internally so version skew is covered
twice, serverConfig resolves from the active thread's environment so
supportsPinning matches the thread being toggled, the effect's dependency array
gained every new closed-over value, shiftKey is compared exactly so the chord
cannot also fire mod+p filePicker.toggle, existing installs pick up the new
default via syncDefaultKeybindingsOnStartup, repeated presses are a server-side
no-op re-emit, and no command-palette or settings map needed a new entry.