fix(cli): /local can override the agent's model so requests don't fail with "model not found"

Without this, `/local on` redirected the URL to Ollama but the agent
still sent its declared cloud model id (e.g. anthropic/claude-opus-4-7)
which Ollama doesn't have — every prompt failed with "model not found".

Adds a CODEBUFF_LOCAL_MODEL env var that overrides the agent's model
when a custom provider is active. The SDK reads it lazily in
promptAiSdkStream, mirroring how the URL is resolved. Override only
applies to agents WITHOUT their own providerOptions.baseUrl — explicit
per-agent config is left alone.

/local subcommands grew:
  /local on <model>            — enable with default URL + model
  /local on <url> <model>      — enable with both
  /local model <name>          — set model after enable
  /local model clear           — drop the override
  /local list                  — probe Ollama's /api/tags for models
  /local <model-with-colon>    — shortcut: same as `on <model>`

The status message also nudges users toward setting a model: without
an override, it warns that cloud model ids will go to the local
provider and fail.

40 unit tests cover: parser shapes (URL only, model only, both,
aliases, bare URL, bare model with-colon shortcut), URL/model
validation, set-model rejection when local is off, idempotent
disable, end-to-end env mutation via the SDK getter.

All 3245 existing tests across CLI + SDK + agent-runtime still pass.

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

Author: vraj00222