feat(projects): make the home channel the project surface

[thomaspblock]

Summary

• render an explicit project's home channel through the normal channel timeline and composer
• add a resizable project context rail with codebase, channel, people, and workspace navigation
• keep project agent conversations bounded to the project home and preserve repository/detail routes

This is Part 4 of the channel-first Projects stack, based on #6594. The final part contains overview and workspace completion polish.

Testing

• focused project conversation, route, summary, workspace-sheet, and related-channel tests: 39/39 passed
• Desktop unit suite: 5,439/5,439 passed
• E2E-mode Desktop build passed
• TypeScript, Biome, and differential file-size checks passed
• full pre-push gate passed

Post-Deploy Monitoring & Validation

• open project homes from project and channel entry points in the first staging Desktop session
• healthy signals: one channel timeline/composer, stable repository context, bounded project agent history, and reversible workspace sheets
• failure signals: duplicate channel surfaces, stale repository selection, unrelated DM history, or sheets replacing the channel route; mitigate by reverting this PR