feat(offload): reasoning offload for codegraph_explore (bring-your-own endpoint)

codegraph_explore can now hand the source it retrieved to a reasoning model you
point at — any OpenAI-compatible endpoint (Cerebras, OpenAI, a local vLLM/Ollama)
with your own key — and return that model's tight, cited answer instead of the
raw source dump. The agent's main context gets the answer in far fewer tokens, at
the cost of one network round-trip.

Off by default. Configure with `codegraph offload set-endpoint <url> --model <m>
--key-env <ENV>` (or the CODEGRAPH_OFFLOAD_* env vars); status/disable manage it.
The API key is never written to disk — the config stores the NAME of an env var
and the key is read from it at call time. Strictly degradable: any failure
(no endpoint, network, timeout, empty answer) returns null and the call falls
back to the local source, so the offload can never surface an error to the agent.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -606,6 +606,44 @@ add a negation — `!vendor/`. The defaults apply uniformly, so committing a
 dependency or build directory doesn't force it into the graph; the `.gitignore`
 negation is the explicit opt-in.
 
+## Reasoning offload (bring your own model)
+
+**Optional, off by default.** Normally `codegraph_explore` returns the verbatim
+source it retrieved and your agent reasons over it. With reasoning offload, that
+source is instead handed to a reasoning model **you** point at, which returns a
+tight, cited answer — so your agent's main context gets the answer, not a wall of
+source. You trade one network round-trip for far fewer main-context tokens.
+
+Point it at **any** OpenAI-compatible endpoint with your own key — Cerebras,
+OpenAI, a local vLLM or Ollama, anything. Nothing but the assembled context + your
+question leaves your machine, and your API key is **never written to disk** (the
+config stores the *name* of an env var; the key is read from it at call time).
+
+```bash
+# Enable — URL ends in /v1; the key is read from the named env var at call time
+codegraph offload set-endpoint https://api.cerebras.ai/v1 \
+    --model gpt-oss-120b --key-env CEREBRAS_API_KEY
+
+codegraph offload status     # show the current endpoint / model / key source
+codegraph offload disable    # turn it back off
+```
+
+Restart your editor/agent session afterward so running MCP servers pick it up.
+Everything is also settable by env (these override the saved config — handy for
+CI): `CODEGRAPH_OFFLOAD_URL`, `_MODEL`, `_KEY`, `_EFFORT` (`low`|`medium`|`high`),
+`_STYLE` (`plain`|`report`).
+
+A few things worth knowing:
+
+- **Quality tracks the model you choose.** The synthesis prompt is correctness-first
+  (it leads with a `Coverage: full / partial / not found` verdict and cites
+  `file:line` for every claim, so answers stay verifiable), but a weak endpoint can
+  still be confidently wrong. It's designed and validated against `gpt-oss-120b`-class
+  models at low temperature.
+- **It's strictly degradable.** Any failure — no endpoint, network error, timeout,
+  empty answer — silently falls back to returning the local source. The offload can
+  never break a call.
+
 ## Telemetry
 
 CodeGraph collects **anonymous usage statistics** — which tools and commands get