review(dream): harden line-age annotation per review feedback

Follow-up to #3212, fully backward compatible:

- Extract the 14-day staleness threshold as `_STALE_THRESHOLD_DAYS` module
  constant and pass it into the Phase 1 prompt template as
  `{{ stale_threshold_days }}`. The number lived in three places before
  (code threshold, prompt instruction, docstring); now there is one.
- Add `DreamConfig.annotate_line_ages` (default True = current behavior)
  and propagate it through `Dream.__init__` and the gateway wiring in
  cli/commands.py. Gives users a knob to disable the feature without a
  code patch if an LLM reacts poorly to the `← Nd` suffix.
- Harden `_annotate_with_ages` against dirty working trees: when HEAD
  blob line count disagrees with the working-tree content length, skip
  annotation entirely instead of assigning ages to the wrong lines. The
  previous `i >= len(ages)` guard only handled one direction of the
  mismatch.
- Inline-comment the `max_iterations` 10→15 bump with a pointer to
  exp002 so future blame has context.
- Add 4 regression tests: end-to-end `← 30d` reaches prompt, 14/15
  threshold boundary, `annotate_line_ages=False` bypasses git entirely
  (verified via `assert_not_called`), length-mismatch defense, and
  template-var rendering.

Made-with: Cursor

nanobot/cli/commands.py

@@ -871,6 +871,7 @@ def gateway(
         agent.dream.model = dream_cfg.model_override
     agent.dream.max_batch_size = dream_cfg.max_batch_size
     agent.dream.max_iterations = dream_cfg.max_iterations
+    agent.dream.annotate_line_ages = dream_cfg.annotate_line_ages
     from nanobot.cron.types import CronJob, CronPayload
     cron.register_system_job(CronJob(
         id="dream",