Collapse the three runtime bootstraps into shared session assembly

[TheGreatAxios]

Summary

• Shared session assembly owns the bootstrap wiring runTUI and runExec used to duplicate (trust, permission gate, chat agent, lifecycle, advertised tools, sources)
• runTUI's closure scope decomposes into prepareTUISession and the plugins-admin backend with explicit inputs
• runSubAgent consumes the shared layer-1 primitives
• Callee option types include explicit | undefined so forwarding needs no conditional spreads

Verification

• bun run typecheck, bun run build, and bun run test pass
• No behavior change: verbatim moves, same call order, live values stay behind getters

Fixes CL-6810

[linear-code]

CL-6810

[TheGreatAxios]

Primary · Approve

Shared assembly plus live getters. /clear rebuilds against the new workdir; crash finalize and compaction mode follow the loop lets.

Residual, not blocking:

• Diff is net +752 (1787/1035), so the issue's 300–500 net-negative box stays open.
• No driven TUI session or headless exec run in this pass — suite only (bun run typecheck / build / test).

[TheGreatAxios]

Critic · Comment

No blocking defects on the tree as it stands. Live getWorkdir / getCompactor run inside buildAgent; crash identity rebinds via bindLiveSession.

Not blocking:

• inactivityTimeoutMs / totalTimeoutMs still snapshot at assemble time. No current path reassigns them before rebuild.
• createPluginsAdmin has no dedicated tests after the extract (origin had none either).