feat: remove reasoning offload / CodeGraph AI managed reasoning feature

Strips the bring-your-own-model reasoning offload and managed CodeGraph AI
integration (login/logout/usage commands, offload config/credentials/reasoner
modules, and the synthesizeOffload call in codegraph_explore). The eval findings
showed raw source output outperformed the synthesized path on accuracy, so
codegraph_explore reverts to returning verbatim retrieved source exclusively.

CHANGELOG and README sections for reasoning offload are removed; test comments
and DEFAULT_MCP_TOOLS description are updated to drop offload references.

README.md

@@ -598,44 +598,6 @@ 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