docs: mark fb-26 + fb-28 merged, HOTFIXES entries for progress bugs + readonly_mode

- fb-26 (progress.rs): Fixed Aborted unconditional writeln (TTY duplicate output)
  and Completed TTY path missing summary line. Added KEBAB_PROGRESS=plain env
  override and quiet field to ProgressMode.
- fb-28 (main.rs): Added --readonly / --quiet global flags with KEBAB_READONLY env.
  Readonly blocks mutating commands (ingest/ingest-file/ingest-stdin/reset) with
  exit code 1; error.v1 code "readonly_mode" in --json mode. Quiet suppresses all
  human progress/hint stderr while preserving errors.
- Updated task spec status for p9-fb-26 and p9-fb-28 to 'merged'.
- Updated tasks/INDEX.md and HANDOFF.md with merge status and summary entries.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tasks/p9/p9-fb-26-ingest-log-consistency.md

@@ -3,7 +3,7 @@ phase: P9
 component: kebab-cli
 task_id: p9-fb-26
 title: "Ingest 로그 출력 일관성 (in-place vs 새 줄 혼재)"
-status: open
+status: merged
 target_version: 0.3.0
 depends_on: [p9-fb-02]
 unblocks: []