CL-7007: Give retained sessions their own retention cap

[TheGreatAxios]

Summary

• Retained worker sessions (spawn_agent, resumable via resume_agent/followup_task) were sharing pruneCompleted's maxCompleted (a TUI display cap, default 20) with every other finished session, a side effect of CL-7002's correct removal of their previous unbounded exemption. Past 20 spawned workers, resume_agent on an early one failed with a bare not_found.
• Adds maxRetained (default 50), a separate cap sized for fan-out, governing only open retained sessions (retained:true, lifecycleStatus completed/interrupted). maxCompleted now excludes these and continues to bound every ordinary finished session exactly as before.
• Eviction under maxRetained picks least-recently-used, never evicts a running session, and still calls releaseHandles (sidecars/reactor/lock entry) exactly like pruneCompleted's eviction.
• An evicted session is tombstoned (bounded to 500 entries) so resume_agent/followup_task/close_agent report an actionable terminal status plus a read_agent_trace pointer instead of not_found.
• Fixed the stale comment on SubAgentSession.retained claiming an exemption from pruneCompleted's cap that no longer exists.
• maxRetained resolution guards non-finite/undefined config (including a JSON round-trip turning a configured Infinity into null) so it can never silently collapse to 0.

Tests changed and why

Three existing tests encoded "retained sessions share the display cap" as correct behavior — that was exactly this ticket's defect, so they were updated rather than left to falsely pass:

• session-store.test.ts: pruneCompleted evicts a retained, still-open session past maxCompleted... → pruneRetained evicts a retained, still-open session past maxRetained.... Added a companion asserting maxCompleted still evicts an ordinary (non-retained) session.
• agent-fleet.test.ts: reports survive well past the session store's display cap (20)... previously asserted the earliest of 25 spawned retained sessions was evicted from the store; now asserts it is not (25 < default maxRetained 50).
• retain-salvage.test.ts: retained completed sessions are exempt from the display cap without bound → ...bounded by maxRetained, not the display cap, now configuring maxRetained instead of relying on maxCompleted.

New tests added (session-store.test.ts): resume_agent on an evicted session returns actionable status + hint; a running session is never evicted by maxRetained; maxRetained bounds memory across 50 spawned-and-completed retained sessions.

Test plan

• bun test src/subagent/session-store.test.ts src/subagent/agent-fleet.test.ts src/subagent/lifecycle-tools.test.ts src/subagent/retain-salvage.test.ts — all pass
• bun run check (lint, typecheck, build, full test suite) — 5294 pass, 0 fail

[linear-code]

CL-7007