feat(agent): fill Skills/MCP/Rules categories in context breakdown

[k11kirky]

Problem

The context breakdown displayed in usage updates was missing accurate token estimates for the Rules (CLAUDE.md), Skills (slash commands), and MCP tools categories. These were left at zero even when content was present, making the breakdown unreliable for understanding what's consuming context.

Changes

Three new estimator functions were added to context-breakdown.ts:

• estimateRulesTokens — reads the raw CLAUDE.md content and estimates its token contribution to the system prompt.
• estimateSkillsTokens — estimates tokens for the slash-command list (name, description, hint) injected by the SDK.
• estimateMcpTokens — estimates tokens for connected MCP tools based on their name and description.

A readClaudeMdQuietly helper reads CLAUDE.md from the project root at session initialization, falling back to undefined on any IO error, so the rules estimate is populated from the start.

A getCachedMcpTools function was added to tool-metadata.ts to expose a snapshot of the current MCP metadata cache for use by the estimator.

A updateBreakdownCategory method was introduced on ClaudeAcpAgent to safely patch a single key of the contextBreakdownBaseline without clobbering other fields. It is called after slash commands are fetched and after MCP tool metadata is loaded, so the baseline stays current as those values become available.

How did you test this?

Unit tests were added for all three new estimator functions covering zero/empty inputs, positive token counts, and scaling behavior. Existing buildBreakdown tests continue to pass.

Publish to changelog?

No

[k11kirky]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat(billing): single-source usage via main-process relay #2358
• feat(agent): fill Skills/MCP/Rules categories in context breakdown #2357 👈 (View in Graphite)
• feat(sessions): context breakdown popover #2353
• feat(agent): emit per-category context token breakdown #2352
• feat(billing): threshold notification service with persisted dedupe #2351
• feat(billing): always-on Free sidebar bar with reset time #2350
• main

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