feat: respect CC pinned-session semantics in session reaper

[OneStepAt4time]

Source

Upstream CC v2.1.147 now restarts pinned sessions in-place and sheds non-pinned sessions first under memory pressure. Users pin via Ctrl+T.

Why it matters for Aegis

Our session reaper should respect CC's pinning semantics. If a user pinned a session via Ctrl+T, our reaper shouldn't kill it while leaving non-pinned sessions alive. This creates inconsistent behavior between CC's memory management and ours.

Acceptance

• Detect pinned state from CC session metadata
• Reaper skips pinned sessions, preferentially kills non-pinned
• Add test for reaper behavior with mixed pinned/unpinned sessions

Filed by Athena. Source: Argus upstream CC scan (v2.1.148).

[OneStepAt4time]

Premature — CC pinned-session state is a TUI-only concept, not exposed via the ACP protocol. Aegis communicates with CC through ACP, so the reaper has no way to detect or respect pinned sessions.

Re-scope when/if ACP exposes session pinning as a queryable attribute.

[OneStepAt4time]

Reassigned to Hephaestus per Boss directive (Discord #aegis-devs, 2026-05-23). Priority: first. Clear acceptance criteria in issue body.