fix(rust): wire ICM compressor into Rust proxy on /v1/messages

Adds an opt-in compression interceptor that buffers Anthropic
/v1/messages requests, runs IntelligentContextManager over the
messages array, and forwards the (possibly trimmed) body upstream.
All other paths, methods, and content-types stay on the original
streaming passthrough — so existing operators see zero change.

Behaviour gates ALL must be true to buffer + compress:
  - --compression flag (or HEADROOM_PROXY_COMPRESSION=1)
  - method == POST
  - path == /v1/messages
  - Content-Type: application/json
  - ICM constructed successfully at startup

Falls through to streaming on any failure: parse, missing fields,
unknown model, body-too-large. Compression must never break a
request — that's the safety contract.

Model context windows come from a vendored LiteLLM snapshot at
crates/headroom-proxy/data/model_prices_and_context_window.json
parsed once into an OnceLock<HashMap>. Refresh via
scripts/refresh_model_limits.sh. Rationale documented inline:
hardcoded tables silently rot; LiteLLM is the canonical source
the entire LLM-tooling ecosystem relies on.

New tests:
  - 16 unit tests across compression::{anthropic, icm, model_limits}
  - 5 integration tests: off-passthrough, on-short-passthrough,
    on-oversized-trim, on-non-json-skip, on-non-llm-path-skip

Verification:
  - cargo test --workspace -> 884 passed, 0 failed
  - cargo clippy --workspace -- -D warnings -> clean
  - cargo fmt --check -> clean

plugins/headroom-agent-hooks/.github/plugin/plugin.json

@@ -1,6 +1,6 @@
 {
   "name": "headroom",
-  "version": "0.20.2",
+  "version": "0.20.8",
   "description": "Headroom startup hooks for Claude Code and GitHub Copilot CLI.",
   "author": {
     "name": "Headroom Contributors",