fix(bash): address code-review findings on background shells

[yogthos]

Post-merge code review of #236 (background-shells rework) surfaced six findings; this fixes #1–#5 and documents #6.

#	Sev	Fix
1	HIGH	spawn_streaming_shell never disarmed its PgKillGuard, so on natural (and timeout) exit the guard's Drop fired a second killpg(SIGKILL) against an already-reaped — possibly OS-recycled — process-group id (could kill an unrelated group). Now disarmed before finish() on every non-aborted completion path, matching run_with_timeout. The guard still fires on the abort path, which is exactly how kill_shell/kill_all kill the group.
2	MED-HIGH	Background shells (own process group) leaked past normal exit — kill_all() only ran on session swap. Now also called when run_interactive returns (/quit, Ctrl+C/D, EOF) and at the headless session-end points.
3	MED	register() eviction at capacity dropped a still-running entry's JoinHandle (tokio detaches on drop → orphaned, untracked process). Now evicts the oldest terminal entry preferentially, and aborts the handle if a running entry must be evicted as a last resort.
4	MED	The background arg schema description still described the reverted push-once model ("output delivered automatically, do NOT poll, killed at the timeout") — which would stop the model from ever polling bash_output. Rewritten to the poll / unbounded / kill_shell model.
5	LOW	Fixed a stale test doc-comment referencing the removed TaskKind::Shell.
6	LOW	Documented the Windows process-tree-kill limitation on spawn_streaming_shell (Unix-only PgKillGuard; grandchildren best-effort on Windows).

Verified-clean in review and left as-is: the abort→SIGKILL chain on unix, drain-EOF-while-process-alive, the cap TOCTOU (bash is Sequential), global-store sharing, /tasks listing, and the status-bar call sites. The soft output-buffer cap and capturing trailing output after exit are intentional.

Tests

• New: eviction aborts (not detaches) an evicted running shell; eviction prefers terminal entries over running ones.
• Full feature-matrix suite green at -D warnings (2166 passed).