fix(agent-runtime): isolate delegate mutation recovery and failure results

[AR307]

Concurrent Task delegates currently share the parent's mutation recovery keys.
Failures from two writers editing the same path can terminate one writer before
its third failure and set the parent's terminal error. A delegate that wrote
introductory text before exhausting recovery can also return completed, even
though its editing work failed. Both failures reproduce on upstream 0.17.0
with a controlled local provider and no fork-specific features.

Scope retry counts and per-code graces to the executing parent or delegate run.
Carry exhaustion into that delegate's failed Task result, preserve its earlier
report and resumable chain, and release its recovery state when it settles.
A new parent prompt resets only the parent's budget. The existing three-failure
limit, permission checks and cancellation precedence remain unchanged.

The shared mutation descriptions are extracted to a small module so the runtime
entry point shrinks despite the added ownership handling.

Reproduction and validation:

• node scripts/e2e-subagent-edit-isolation.mjs: real Runtime, pi Agent and
  SubagentRun over local HTTP/SSE. On upstream, A terminates after only two
  failed Edits as B spends the shared budget. After the fix, A fails on its own
  third attempt, B completes after two failures, and the parent has no mutation
  error. Resuming A succeeds.
• --single: upstream reports A as completed and reports
  MUTATION_RETRY_BUDGET_EXHAUSTED on the parent. The fixed result is failed
  on A, without the parent error; resume succeeds.
• --patch: the same isolation and resume flow passes for shell patch commands.
• Runtime and subagent regression suites: 327 passed, including parent restart
  with a still-running delegate under default and explicit permissions.
• Agent-runtime TypeScript build: passed.

All three provider-boundary scenarios and the 327 regressions were also run on
this standalone PR branch. Only provider responses and the host tool boundary
are controlled fixtures; no paid provider or user profile is used.

Related work: #1193 classifies output-token truncation, #1394 preserves resume after a parent error, and #283 publishes settlement updates. This change fixes per-delegate mutation recovery ownership and exhaustion classification.

Validation candidate: 7a03669b0f0c661f75c26e7b463b0a7f50dd4272; upstream main: 72b5e826cb7a9928467091ccf745aa9b225eeb04.

[muzimu217]

Verified locally on head 7a03669b0: packages/agent-runtime runtime.test.ts passes 287/287 (including the expanded coverage), full pnpm build:js clean.

The scoping mechanism matches the described fix:

• delegateMutationTerminations keys terminal-failure state by toolCall.id, so concurrent delegates no longer share — and can no longer prematurely exhaust — the parent's recovery budget; entries are released when a delegate settles and the map is cleared on a new parent prompt, which keeps the "three failures" semantics per run instead of per session.
• Carrying exhaustion into the delegate's own failed Task result while preserving its earlier report and resumable chain fixes the completed-despite-failed-editing case the body describes, and the honest-retry exemption set (EDIT_TAG_MISMATCH / EDIT_TAG_UNKNOWN / EDIT_LINES_UNSEEN) is consistent with the line-anchored edit contract's §9.3 reveal semantics.
• Both failure modes say they reproduce on unmodified 0.17.0, and scripts/e2e-subagent-edit-isolation.mjs covers the isolation end-to-end — the right regression shape for a concurrency bug.

Non-blocking notes: the mutationTerminationAdvice strings are a nice touch (they point the model at the specific recovery action per error code); worth watching that the advice set stays in sync with the recoverable-codes set as both evolve.

Nothing blocking from a correctness standpoint; fix scope fits the current contribution window.