fix(compress): gate N-user tail guarantee to actionable turns, behavior-preserving default

Follow-up fixes on top of the salvaged #22566 mechanism:

- N-collector now counts only REAL actionable user turns via
  _is_actionable_user_turn + _is_synthetic_compression_user_turn —
  the same filter pair _find_last_user_message_idx uses post-#69291.
  The contributor's bare role=='user' + _is_context_summary_content
  check let blank platform echoes and continuation/todo rows consume
  N slots, silently degrading the guarantee.
- Default flipped 3 -> 1 (behavior-preserving): a default of 3 was
  measured to change the tail cut on transcripts whose budget covers
  only the last turn. min_tail_user_messages=1 delegates to the
  existing single-user anchor; N>1 is opt-in, and the call site is
  gated so the default path is byte-identical to main.
- Hardened config parse in agent_init (bool rejected, fractional
  floats rejected, floor 1) matching the max_attempts parser shape.
- Wired the recurring external-PR config gaps: hermes_cli/config.py
  DEFAULT_CONFIG + cli-config.yaml.example (PR only had cli.py).
- Regression tests: blank echoes / synthetic rows don't count toward
  N; tool-call/result pairs never split by the N-boundary (no-orphan
  both directions); N-guarantee wins over tail_token_budget and the
  _MAX_TAIL_MESSAGE_FLOOR (floor is a minimum, not a cap); default
  parity pin; DEFAULT_CONFIG pin.

cli.py

@@ -468,7 +468,7 @@ def load_cli_config() -> Dict[str, Any]:
         "compression": {
             "enabled": True,      # Auto-compress when approaching context limit
             "threshold": 0.50,    # Compress at 50% of model's context limit
-            "min_tail_user_messages": 3,  # Min recent user messages to preserve in tail after compression
+            "min_tail_user_messages": 1,  # Real user messages guaranteed in the tail (1 = existing single anchor)
         },
         "agent": {
             "max_turns": 90,  # Default max tool-calling iterations (shared with subagents)