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.

packages/core/agent/src/types.ts

@@ -206,6 +206,11 @@ declare module 'cordis' {
      * summarization model call).
      * @mode serial
      */
+    // TODO: `fullSystemPrompt` is a smell on a generic per-step seam — compaction
+    // is its only consumer, so a wide event carries a string just one listener
+    // reads. Revisit if no second consumer appears: e.g. hand listeners a lazy
+    // prompt provider, or move token-pressure measurement behind a
+    // compaction-specific seam instead of the shared pre-step checkpoint.
     'agent/pre-step'(agent: Agent, turn: number, step: number, fullSystemPrompt: string, signal: AbortSignal): Promise<void> | void
     /**
      * Waterfall: mutate the fully-assembled {@link GenerateOptions} before the