Agent2@narko: feat(run): report a local model's context window in the result

[muxagent]

Summary

AgentMux's agent pane shows a context meter, "tokens in context / window". It already reads muxcode's per-call usage from message_start / assistant frames, but muxcode never reports a window, so a muxcode pane shows "N ctx" with no window and no fill band.

The result frame now carries Claude Code's own field when the window is known:

"modelUsage": { "<model>": { "contextWindow": 32768 } }

• Local models: the window is the context size llama-server is started with. That is the model's context_window from its metadata, else 4096. A new contextSizeFor(modelPath) in llama-server/manager.ts is now shared by the server launch and LocalBackend.contextWindow, so the reported window is exactly the one in effect.
• Model ids: the window is keyed by the backend's model id and by every id message_start carried, since a server may answer under another id. AgentMux matches the window to message.model.
• Remote backends (OpenAI-compatible, Anthropic) report nothing, as before. muxcode doesn't know their windows, and AgentMux shows an unknown window as unknown.

AgentMux side, for context: the meter takes the window from result.modelUsage[*].contextWindow (agentmuxai/agentmux frontend/app/store/agent-pane-state/context-reading.ts).

Tests

• New test/context-window.test.mjs exercises the built emitter:
  • no window: no modelUsage;
  • a local window under both ids;
  • an error result carries it too.
• npm test: 81 pass, 1 skipped (as before).

Agent: Agent2@narko

🤖 Generated with Claude Code

[muxreview]

ReAgent Diagnostics

Field	Value
ReAgent Version	5.34.5+gad210a0
Trigger	PR opened
Tier	STANDARD
Project Context	No CLAUDE.md
Model	claude-sonnet-5-5
Effort	high
Ref Repos	Enabled (dev-tools, shared-infrastructure)
Merge Safety	No recent-code deletions
Review Time	8s
Timestamp	2026-10-06T23:29:29Z
Repository	agentmuxai/muxcode
PR	#52

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Issues:

• [P1] package.json:3 - Missing version bump for code changes (new feature in src/, no diff found for the "version" field; the repo has no changeset directory, so each source PR carries its own minor bump)

[muxreview]

ReAgent Diagnostics

Field	Value
ReAgent Version	5.34.5+gad210a0
Trigger	Re-review (new commits)
Tier	VERSION_CRITICAL
Project Context	No CLAUDE.md
Model	claude-sonnet-5-5
Effort	high
Ref Repos	Enabled (dev-tools, shared-infrastructure)
Merge Safety	No recent-code deletions
Review Time	5s
Timestamp	2026-10-06T23:30:13Z
Repository	agentmuxai/muxcode
PR	#52

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LGTM

[a5af]

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