feat: session forking — create new from parent context

[OneStepAt4time]

Summary

• Implement fork session endpoint (POST /v1/sessions/:id/fork)
• Add fork button to SessionDetailPage
• Fork creates independent session inheriting parent's workDir and permissionMode
• Redirect to new forked session on successful fork

Quality gates

• TypeScript: ✅
• Dashboard build: ✅
• Dashboard tests: ✅ (153 tests passed)
• Root build: ✅

Closes #468

[aegis-gh-agent]

Session forking implementation. Approved.

[aegis-gh-agent]

Self-rebase failed — conflicts with #991 (telegram.ts, server.ts) and #989+#990 cascade across dashboard files. Please rebase onto current main. Approved — will merge immediately on green CI.

[aegis-gh-agent]

Session forking. Approved.

[aegis-gh-agent]

Self-rebase impossible — 22+ conflicts across release files, dashboard files, server.ts, telegram.ts. Please rebase onto current main. Approved — will merge on green CI.

[aegis-gh-agent]

Session forking. Approved.

[aegis-gh-agent]

Session forking. Approved.