Consolidate stall-watchdog's repetition detector into subagent/repetition.ts

[TheGreatAxios]

Part of CL-6939 (item 3 only) and CL-6790 — neither issue should auto-close on this merge.

Summary

CL-6939 / CL-6790: src/tui/stall-watchdog.ts carried its own char-level tail-repetition detector (four constants — REPETITION_MIN_PERIOD, REPETITION_MIN_REPEATS, REPETITION_MAX_PERIOD_CAP, REPETITION_MIN_DISTINCT_CHARS — plus a thin wrapper over detectSequencePeriod), duplicating a detector that conceptually belongs next to src/subagent/repetition.ts's existing degenerate-repetition guard.

• Moved the constants, doc comments, and the wrapper function into src/subagent/repetition.ts, renamed to detectTailCharLoop / TailCharLoopCheck (constants keep the CHAR_ prefix) so it reads clearly next to that module's pre-existing detectRepetition (a different, KMP + digit-folding detector for streamed model text) rather than shadowing its name.
• src/tui/stall-watchdog.ts no longer re-exports anything for this — the old detectRepetition / RepetitionCheck names are gone from that file entirely.
• The two real consumers now import detectTailCharLoop directly from src/subagent/repetition.js: src/tui/turn-state.ts and src/tui/stall-watchdog.test.ts. Verified with grep that nothing still imports the old names from stall-watchdog.js.
• src/util/period-detection.ts (the generic detectSequencePeriod primitive) is untouched — it's the shared engine detectTailCharLoop delegates to, same as before.

No threshold value changed. Every constant (8, 24, 2000, 8) carries over unchanged.

Not merged into one function: subagent/repetition.ts's pre-existing detectRepetition (used by run.ts/summarizer.ts) and the newly-added detectTailCharLoop (used by the TUI) solve the same "is the tail looping" question with genuinely different algorithms — normalize + KMP + optional digit-folding for streamed model text vs. a plain per-character search with a distinct-chars floor for the TUI's live buffer. Keeping them as two functions in one module avoids retuning either to match the other, per the standing no-threshold-retuning rule.

Net line delta: -9 (git diff --stat main...branch: +22 insertions, -31 deletions across src/subagent/repetition.ts, src/tui/stall-watchdog.ts, src/tui/stall-watchdog.test.ts, src/tui/turn-state.ts).

Test plan

• bun test src/tui/stall-watchdog.test.ts src/subagent/repetition.test.ts src/tui/turn-state.test.ts — 99 pass, all existing coverage preserved (no tests deleted)
• bunx tsc --noEmit clean
• bun run check — 5277 pass, 0 fail

[linear-code]

CL-6939