CL-6946 part 2: delete prose-based sub-agent stop-reason matchers

[TheGreatAxios]

Summary

Threads the structured ForcedStopReason through the sub-agent dispatch path instead of re-deriving it by parsing the returned report's prose.

• runSubAgent now returns { report, stopReason? } instead of a bare string; stopReason is set whenever the run ends via a forced stop (turn budget, no-progress, never-acted, never-edited, deadline, cancelled, stalled, repetition, incomplete-report).
• SubAgentDirector exposes the reason via observeForcedStop, wired into runSubAgent's return.
• task-tool.ts classifies the salvage outcome and selects the parent hint from result.stopReason directly, and stamps ToolResult.detail.stopReason so the parent chat director can do the same without parsing tool-result content.
• classifyBriefSalvage is now (input: { stopReason?, wasCancelled }) => BriefSalvageKind | null — a trivial passthrough, no string/regex matching anywhere.
• appendSubAgentParentHints now switches on the structured reason instead of re-parsing report text.
• src/agent/director.ts's hard-block-salvage nudge (previously classifyBriefSalvage(toolResultContentString)) now reads the same structured detail.stopReason off the tool result.

Deleted

• isForcedStopSubAgentReport and every per-reason wrapper (isTurnBudgetSubAgentReport, isNeverActedSubAgentReport, isNeverEditedSubAgentReport, isDeadlineSubAgentReport, isRepetitionSubAgentReport, isNoShipSubAgentReport, isNoProgressSubAgentReport, isStalledSubAgentReport, isCancelledSubAgentReport, isIncompleteReportSubAgentReport).
• appendTurnBudgetParentHint, appendNeverActedParentHint, appendNeverEditedParentHint, appendDeadlineParentHint, appendNoShipParentHint, appendRepetitionParentHint, appendNoProgressParentHint (folded into one structured switch in appendSubAgentParentHints).
• parseSubAgentReport import from brief-dispatch.ts's consumer path (the genuinely different completeness-gate use of hasReportEnvelope in stop-policy.ts — checking the model's own final turn for a report envelope as a nudge trigger — is unrelated and kept).
• Tests that asserted prose-matching behavior of the removed functions; replaced with tests proving classifyBriefSalvage decides from the structured value with no text involved.

Not touched

The sibling lanes (CL-6994, CL-6995) hadn't landed on main as of this branch, so nothing described in the ticket was already removed — all the ForcedStopReason variants and their prose renderers in FORCED_STOP_SUMMARIES are still present; only the classification path changed.

Net diff

git diff --stat origin/main...HEAD: 18 files changed, 302 insertions(+), 339 deletions(-) — net −37 lines.

Test plan

• bun test src/subagent/index.test.ts src/agent/director.test.ts src/subagent/run-authority.test.ts src/subagent/agent-fleet.test.ts src/subagent/task-tool-worktree.test.ts src/perf/permission-subagent-spans.test.ts tests/unit/subagent-session-store.test.ts tests/unit/subagent.test.ts tests/unit/telemetry-product-events.test.ts — 250 pass, 0 fail
• bunx tsc --noEmit — clean
• bun run check — lint/typecheck/build pass; 8 pre-existing failures in unrelated integration suites (git-push-scoped, late-MCP-dispatch, reactor-permission-multi-turn) — timing/network flakes untouched by this change

[linear-code]

CL-6946