fix(compression): include system prompt + tool schemas in token estimates (#18265)

The user-visible /compress banner and the post-compression last_prompt_tokens
writeback both counted only the raw message transcript (chars/4). With a 15KB
system prompt and 30 tool schemas (~26KB), a 4-message transcript that looks
like ~45 tokens to the transcript-only estimator is really ~10.5K tokens of
request pressure — a 234x gap.

Two user-facing consequences:
- Banner shows 'Compressing … (~45 tokens)…' while compression is actually
  firing on 10K+ tokens of real pressure, confusing users about why
  compression triggered (reported by @codecovenant on X; #6217).
- Post-compression last_prompt_tokens writeback omits tool schemas, so the
  next should_compress() check compares real usage against a stale
  underestimate — compression triggers late, potentially past the model's
  context limit on small-context models (#14695).

Swap estimate_messages_tokens_rough() for estimate_request_tokens_rough()
at every user-visible banner and at the post-compression writeback.
estimate_request_tokens_rough() already existed for exactly this purpose
and includes system prompt + tool schemas.

Touched call sites:
- run_agent.py: post-compression last_prompt_tokens writeback, post-tool
  call should_compress() fallback when provider usage is missing
- cli.py: /compress banner + summary
- gateway/run.py: gateway /compress banner + summary
- tui_gateway/server.py: TUI /compress status + summary
- acp_adapter/server.py: ACP /compact before/after

Left intentionally alone:
- Session-hygiene fallback and the 'no agent' /status path in gateway/run.py
  — no agent instance is in scope to query for system prompt/tools, and the
  existing 30-50% overestimate wobble on hygiene is safety-accepted.
- Verbose-mode 'Request size' logging — informational only, already counts
  system prompt via api_messages[0].

Also relabels the feedback line from 'Rough transcript estimate' to
'Approx request size' so the metric label matches what it actually measures.

Credits: diagnoses from @devilardis (#14695) and @Jackten (#6217);
user report @codecovenant on X (2026-04-30).

Closes #14695
Closes #6217

acp_adapter/server.py

@@ -1068,10 +1068,16 @@ class HermesACPAgent(acp.Agent):
             if not hasattr(agent, "_compress_context"):
                 return "Context compression not available for this agent."
 
-            from agent.model_metadata import estimate_messages_tokens_rough
+            from agent.model_metadata import estimate_request_tokens_rough
 
             original_count = len(state.history)
-            approx_tokens = estimate_messages_tokens_rough(state.history)
+            # Include system prompt + tool schemas so the figure reflects real
+            # request pressure, not a transcript-only underestimate (#6217).
+            _sys_prompt = getattr(agent, "_cached_system_prompt", "") or ""
+            _tools = getattr(agent, "tools", None) or None
+            approx_tokens = estimate_request_tokens_rough(
+                state.history, system_prompt=_sys_prompt, tools=_tools
+            )
             original_session_db = getattr(agent, "_session_db", None)
 
             try:
@@ -1091,7 +1097,13 @@ class HermesACPAgent(acp.Agent):
             self.session_manager.save_session(state.session_id)
 
             new_count = len(state.history)
-            new_tokens = estimate_messages_tokens_rough(state.history)
+            _sys_prompt_after = getattr(agent, "_cached_system_prompt", "") or _sys_prompt
+            _tools_after = getattr(agent, "tools", None) or _tools
+            new_tokens = estimate_request_tokens_rough(
+                state.history,
+                system_prompt=_sys_prompt_after,
+                tools=_tools_after,
+            )
             return (
                 f"Context compressed: {original_count} -> {new_count} messages\n"
                 f"~{approx_tokens:,} -> ~{new_tokens:,} tokens"