feat(web): add file context actions across diff and browser

[rynfar]

What changed

File rows in the diff, changed-files tree, and file browser now offer Open, Reveal, and Open with actions when the selected environment advertises those capabilities. Actions use the existing environment-scoped shell.openInEditor RPC, resolve paths within the active workspace, and omit file operations for deleted entries. The file browser keeps Copy mention and Add to chat.

A menu opened before a workspace or environment switch cannot dispatch into the old context after its asynchronous selection resolves. The committed scope token also rejects a switch away and back, or unmount. Diff headers expose the menu through right-click and Shift+F10/ContextMenu from their existing focusable controls. Browser fallback menus and remote environment RPCs use the same path where available.

Partial adoption of upstream 9051ed9c3923b7b5fb88d99fe9267380d69baa51, adapted to Pylon’s existing editor dispatch and browser fallback. No live browser verification was performed.

Validation

• 69 focused web tests; web typecheck and scoped lint passed (pre-existing lint warnings).
• Formatter and staged diff checks passed.
• Separately reconciled this patch against PR feat(files): browse workspace folders on demand #824’s lazy FileBrowser source in an isolated scratch tree: 71 focused tests and web typecheck passed on the integration candidate. The only source reconciliation was the FileBrowser import/placement.
• Independent adversarial review passed on the final production diff and the last test-only assertion.

Model: GPT-6 Sol (low) via Codex harness, with root agent oversight.

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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	14.0 KiB	13.9 KiB	−62 B (−0.4%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.2 KiB	7.2 KiB	+6 B (+0.1%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.8 KiB	6.7 KiB	−68 B (−1.0%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	58.0 KiB	58.0 KiB	−88 B (−0.1%)	66.4 KiB	✅
Codex	Live turn messages	10	8	−2 (−20.0%)	21	✅
Claude	Total thread wire	14.0 KiB	13.9 KiB	−22 B (−0.2%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.2 KiB	7.2 KiB	−19 B (−0.3%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.7 KiB	6.7 KiB	−3 B (−0.0%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	58.9 KiB	58.9 KiB	−44 B (−0.1%)	66.4 KiB	✅
Claude	Live turn messages	10	9	−1 (−10.0%)	21	✅

Baseline: 7ed3f71 · PR result: 728a7e2 · 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: 115.7 KiB
• Claude decoded thread snapshot: 116.3 KiB

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