Trim HOW/guarantee comments in src/subagent (CL-7009)

[TheGreatAxios]

Comment-only cleanup of src/subagent/*. 25 files, +141/−156. No logic changes.

Closes CL-7009

Why

Nine times in one night a comment asserting a mechanism that did not exist caused real damage. Measured against ~/abklabs/interchange as the house reference:

	comment lines	assert a guarantee	rate
interchange	41,463	83	0.20%
corbits-code	18,978	1,346	7.09%

Interchange is denser in comments than we are (37.8% vs 26% of non-test lines) and still 35× cleaner on this axis, so volume was never the target — the WHY/HOW split is.

Two confirmed false guarantees removed

• session-store.ts — the claim that "completed" is exempt from pruneCompleted's cap. Verified against pruneCompleted(), which has no retained check at all. (CL-7007: Give retained sessions their own retention cap #628 fixed the underlying behavior separately under CL-7007; that file's version on main wins here.)
• agent-fleet.ts — fleetRecords (never capped), directly contradicted by MAX_FLEET_RECORDS = 200 and the eviction logic beneath it.

On the modest count change

The guarantee-grep count moved only a little per file (e.g. run.ts 19→18). That is the honest result rather than a miss: spot-checking the retained matches shows most are genuine WHY-narration that happens to contain "never"/"cannot", not false guarantees — for example silence alone cannot tell "wedged" from "running a ten-minute test suite", and Running sessions are never pruned by this bound, which was verified true against pruneCompleted's status !== "running" && lifecycleStatus !== "running" filter.

The vocabulary grep is a finding aid, not the metric. Driving it to interchange's 0.2% by deleting accurate rationale would make the codebase worse.

Flagged, deliberately not fixed here

1. retain-salvage.test.ts — a test titled "…without bound" whose body asserts a bounded length. Same disease in a test name rather than a comment. CL-7007: Give retained sessions their own retention cap #628 renamed this file's tests; confirm it is resolved there.
2. authority.ts — module doc admits the subtree-authority rule is "not enforced at runtime yet." Pre-existing; worth confirming that statement is itself still true.
3. Missing-test candidate: the "always disposes salvage before reporting resumable" invariant on the deadline/cancel path in run.ts is only covered indirectly. That is the shape a real guarantee should take — a test, not a sentence.

Rebase note

Three files (session-store.ts, session-store.test.ts, agent-fleet.test.ts) conflicted with #628, which rewrote them. Main's versions were taken for those; the cleanup lands in the other 25 files.

Gate: bun run check green — 5294 pass, 0 fail. No CHANGELOG entry: comment-only.

[linear-code]

CL-7009