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.

website/docs/developer-guide/context-compression-and-caching.md

@@ -87,6 +87,7 @@ compression:
   #   "claude-sonnet": 0.35  # overrides" below.
   target_ratio: 0.20         # How much of threshold to keep as tail (default: 0.20)
   protect_last_n: 20         # Minimum protected tail messages (default: 20)
+  min_tail_user_messages: 1  # Real user messages guaranteed in the tail (default: 1)
   codex_gpt55_autoraise: true  # gpt-5.5 on Codex OAuth: raise trigger to 85% (default: true)
   codex_gpt55_autoraise_notice: true  # Show the one-time autoraise notice (default: true)
   codex_app_server_auto: native  # native|hermes|off for Codex app-server thread compaction
@@ -107,6 +108,7 @@ auxiliary:
 | `model_thresholds` | `{}` | map | Per-model overrides of `threshold`. Keys are substring-matched against the model name (longest match wins). The small-context floor still applies on top (see below) |
 | `target_ratio` | `0.20` | 0.10-0.80 | Controls tail protection token budget: `threshold_tokens × target_ratio` |
 | `protect_last_n` | `20` | ≥1 | Minimum number of recent messages always preserved |
+| `min_tail_user_messages` | `1` | ≥1 | Minimum number of REAL (actionable) user messages guaranteed to survive in the uncompressed tail. `1` = the existing single last-user anchor (behavior-preserving default). Raise to e.g. `3` to keep the last 3 real user turns verbatim even when bulky tool outputs fill the tail token budget. Blank platform echoes, compaction handoffs, and synthetic continuation rows never count toward N. The guarantee wins over the tail token budget — the tail may exceed the budget when the anchor pulls the cut back |
 | `protect_first_n` | `3` | (hardcoded) | System prompt + first exchange always preserved |
 | `idle_compact_after_seconds` | `0` | ≥0 seconds | Opt-in: compact up front when a session resumes after this many seconds idle (0 = disabled). Skips when context ≤ threshold × target_ratio; honors cooldown/anti-thrash/lock guards |
 | `codex_gpt55_autoraise` | `true` | bool | Raise the trigger to 85% for gpt-5.5 on the ChatGPT Codex OAuth route (see below). Set `false` to keep the global `threshold` |