feat(helexa-acp): context compaction for small-context local models

A new src/compaction.rs module projects rolling conversation history
into a token budget before each completion. Older tool results and
assistant prose get elided to one-line markers; system prompts, user
turns, and the last KEEP_TAIL=4 messages stay verbatim. tool_call_id
pairing is preserved so OpenAI strict-schema providers keep working.

Driven by a new per-endpoint `context_window` config field (also
HELEXA_ACP_CONTEXT_WINDOW for the env-only single-endpoint case).
When set, prompt budget = context_window - max_tokens - 512_safety;
when unset, behaviour is unchanged.

Without this, a 32 K Qwen3 dies with `prompt_too_long` after the
first few read_file results pile up in history — the symptom seen
in plan-mode dogfooding on beat.

10 new unit tests cover the compaction strategy and the prompt
budget arithmetic.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/helexa-acp/src/main.rs

@@ -16,6 +16,7 @@ use agent_client_protocol::{Result, Stdio};
 use std::sync::Arc;
 
 mod agent;
+mod compaction;
 mod config;
 mod prompt;
 mod provider;