Orchestrator reports why a child stopped, not just that it stopped

[TheGreatAxios]

Problem

When a child agent is terminated by the repetition guard, stall watchdog, salvage caps, or operator cancel, the parent only learned "it stopped" — triage required reading raw trace files.

What already existed

Most of the plumbing was in place: forcedStopReport builds a full Summary/Findings/Blockers/Paths salvage envelope per stop class, the repetition path already led Findings with the looped window snippet + repeat count, and appendSubAgentParentHints stacks per-class re-dispatch hints. The gaps were (1) no machine-readable reason field — only prose in Summary, (2) the child's terminal state (SubAgentSession) carried no reason at all for forced stops that land as status "done", and (3) the TUI's finished-lane transcript row printed a bare "done"/"cancelled".

What this adds (minimal diff on top of the existing plumbing)

• SubAgentReport gains an optional stopped field, rendered as a dedicated first line of the envelope: Stopped: repetition — window "Groaning. " × 1363. Survives the parse/format normalization round-trip; a nested salvage report quoted under Findings cannot leak its own line as the outer reason.
• Each termination path attaches a distinct reason + detail: repetition (window snippet × repeat count), stall abort (seconds quiet after the nudge), deadline (ms elapsed), turn-budget (turns used/max), no-progress (identical-call count), operator cancel (recorded cancel reason). Thrash / no-ship / never-acted / never-edited / incomplete-report carry their bare reason token.
• SubAgentSession.stopReason records the reason on the child's terminal state — parsed from the report's Stopped line on complete(), and set to cancelled — <reason> on cancel(). The bare pre-progress cancel message to the parent now includes the recorded reason too.
• The fleet transcript row for a finished child (observeFleet) announces <lane> stopped — <reason> as an attention line instead of a silent "done" or a bare "cancelled".

Tests

• Stopped-line propagation for repetition (window × count), cancel, and turn-budget through forcedStopReport → parse/format round-trip.
• Session store: complete() captures the report's reason; cancel() records the cancel reason; clean completes stay reason-free.
• Task tool: pre-progress cancel surfaces the recorded reason to the parent.
• Fleet report: stopped-done and cancelled lanes announce the reason.

tsc --noEmit, build, and the full suite (5122 pass / 0 fail) are green. The repo-wide prettier --check / eslint drift in bun run lint is pre-existing on main (the CI lint job already fails there); changed files introduce no new findings.

Fixes CL-6903

[linear-code]

CL-6903