Consolidate duplicated kill-ring and timeout-race modules (CL-6790)

[TheGreatAxios]

Summary

CL-6790 dedup pass. Net line delta (excluding tests/docs): -119 lines (60 insertions, 179 deletions across non-test files).

Consolidated

1. Kill rings — deleted src/tui/kill-ring.ts (the Ink-era copy) and its test. It had zero production callers; src/tui/prompt-kill-ring.ts (used by shell.ts) is the sole, more complete implementation (it also has the killedTextForward/killedTextBackward diff helpers the deleted copy lacked).
2. OAuth helpers — already consolidated in this codebase: the Codex and xAI OAuth modules already import buildAuthorizeUrl/baseTokensFromResponse/exchangeCode/refreshTokenRequest from the generic src/auth/oauth/client.ts and only add provider-specific mapping. No changes needed.
3. REASONING_EFFORTS — already consolidated: src/provider/reasoning-effort.ts re-exports the constant from src/agent/profile-types.ts. No changes needed.
4. mapReactorLike — runtime-bridge.ts now re-exports mapReactorLike from stream-event-map.ts directly instead of wrapping it in an identical local function.
5. Timeout race — extracted src/util/budget-race.ts (BUDGET_EXPIRED, budgetExpiry, withTimeout), consumed by both src/plugins/shell-guard-plugin.ts and src/tui/tool-execution-watchdog.ts, replacing two independent copies of the same AbortController+setTimeout race. Both now reference the shared TIMEOUT_PREFIX constant (exported from tool-time-budget.ts) instead of a hardcoded "[timed out before completing]" string.

Stopped / not touched

• Atomic-write chain (item 6) — the low-level atomicWrite (mkdir + temp file + rename) is already shared: src/workflows/state.ts imports it from src/session/state.ts. The surrounding write-chaining wrapper genuinely differs in behavior between the two: session state's chain is keyed by sessionId, gates writes on isCrashed(), and deletes its map entry once the chain drains; workflow state's chain is keyed by path, has no crash gate, and never evicts entries (relying on flushWorkflowStateWrites for explicit awaiting). Forcing these into one helper would either add a crash gate to workflow writes or drop cleanup from session writes — both are behavior changes, so left as-is per the "stop and report" instruction.
• detectRepetition (two copies) — explicitly out of scope per the ticket; a sibling lane (CL-6995) is removing repetition detection entirely. Not touched (src/subagent/repetition.ts, src/util/period-detection.ts, TUI stall watchdog repetition code all untouched).

Follow-up (not done in this pass)

• A CI check that fails on duplicate exports was requested by the ticket but is out of scope for this pass.

Test plan

• Targeted bun test runs across all touched modules pass.
• bun run check (lint + typecheck + build + full test suite) passes: 5444 pass, 0 fail.
• No changes under vendor/.

[linear-code]

CL-6790