fix: [ENG-2884] address PR #689 codex review

Five items from the latest codex review batch on PR #689.

- agent-process.ts: drop the dead-code `?? ''` fallback on
  previousTexts. archived[] paths are invariant-guaranteed to have
  all three fields (text + mtime + signals) populated together, so
  the fallback was already unreachable — and if it ever fired, the
  resulting `{[path]: undefined}` would violate the new PRUNE
  schema's z.record(z.string(), z.number()) for previousMtimes. All
  three lookups now agree: trust the invariant uniformly.

- search-knowledge-service.ts: extend refreshIndex docstring with a
  maintainer note explaining that the explicit `state.buildingPromise
  = undefined` after the await is defensive (acquireIndex's finally
  block already nulls it). Also documents the benign double-build
  inefficiency when a fresh acquireIndex() races into the gap between
  await-resolve and clear — not a correctness issue, just non-obvious.

- SKILL.md §7: soften the merge bullet's path-attribute warning. The
  prior "drop the trailing .html — the writer always appends it"
  instruction was load-bearing before the path-doubling fix shipped;
  now that the writer normalizes idempotently (both bare and
  .html-suffixed forms resolve to the same on-disk file), the
  instruction reduces to agent-facing cognitive load. Updated to
  "either form works; the writer normalizes" so the agent doesn't
  have to track which form scan emitted.

- dream.test.ts: add two regression tests for the DREAM_REMOVED_FLAGS
  wire-up in dream.ts's run() body. (1) text mode: --timeout 30
  rejects with this.error throwing before this.log runs; (2) json
  mode: --timeout 30 --format json emits a JSON envelope with
  success:false / data.status:'error' / data.error containing
  '--timeout'. Pins the early-exit ordering so a future refactor
  that drops the findRemovedFlagMessage(this.argv, ...) call fails
  the suite instead of silently regressing to pre-removal state.

- schemas.ts: hoist TaskTypeSchema declaration above
  TaskExecuteSchema (the schema previously sat ~100 lines below) and
  replace TaskExecuteSchema.type's inline z.enum(...) with a
  reference to TaskTypeSchema. The doc block at the original
  declaration site claimed it was the single source of truth, but
  the inline enum on TaskExecuteSchema.type silently violated that
  — the only outlier; TaskCreatedSchema.type (now line 605) and
  TaskCreateRequestSchema.type already referenced TaskTypeSchema.
  Hoist needed to avoid a Zod TDZ at module load. No behavioral
  change; schemas now share one literal enum source.