fix(compact): make config knobs explicit and flag two review smells

Address @tianyicui's minor-revision review on PR #110:

- Make every BasicCompactConfig knob required except `auto` (defaults
  true): there is no data yet to justify default thresholds/budgets, so
  a consumer states each value explicitly. Drop the DEFAULTS export and
  the constructor's `= {}` default; example cordis.yml, the compaction
  e2e, the README, and every test construction site now pass a complete
  config (tests route through a `cfg()` helper).
- Add a TODO on estimateContentTokens: char/4 is coarse; replace with a
  real tokenizer or post-response usage feedback in a follow-up.
- Add a TODO on the agent/pre-step `fullSystemPrompt` param flagging it
  as a smell on a generic per-step seam (compaction is its sole
  consumer); a `//` line comment so it stays out of the generated catalog.

examples/coding-agent/tests/compaction.e2e.ts

@@ -50,7 +50,9 @@ describe.skipIf(!process.env.DEEPSEEK_API_KEY)('compaction: a long session compa
         contextWindow: 2400,
         thresholdRatio: 0.5,
         retainTokens: 500,
+        summarizationModel: '',
         maxTokens: 2048,
+        compactionRetries: 1,
       },
       persistenceRoot: './.sessions',
     })