feat(core): structure system context updates - #35045

Summary

• Structures SystemContext reconciliation output so core can report per-source update metadata alongside the existing joined model-visible text.
• Adds descriptions for SystemContext producers and item-level changes for list-shaped guidance sources.
• Keeps the current context-checkpoint event payload unchanged; schema/UI guidance event work remains for WP-F.

Changes

• packages/core/src/system-context: adds source descriptions, structured reconcile updates, structured update normalization, and diffItems reuse for keyed list diffs.
• packages/core/src/{instruction-context,system-context/builtins,session/context-entry}: supplies source descriptions.
• packages/core/src/{skill,reference,mcp}/guidance.ts: supplies source descriptions and item-level structured updates from existing diffByKey results.
• packages/core/test: covers structured updates, item updates, removal metadata, and updated runner checkpoint snapshots.

Decisions / Deferrals

• Source descriptions are validated with the same non-empty text guard as rendered context.
• Removal-capable sources persist descriptions with their removal messages so removed-source updates can still be described after the source disappears.
• context-checkpoint.ts publish payload is intentionally unchanged in WP-C; WP-F owns the session.guidance.updated schema/UI event swap.
• No generated clients were regenerated on this branch.

Part of #35017
Part of #35021

Test Plan

• cd packages/core && bun typecheck
• cd packages/core && bun run test -- test/system-context/index.test.ts test/instruction-context.test.ts test/session-runner.test.ts
• bunx prettier --check packages/core/src/system-context/index.ts packages/core/src/skill/guidance.ts packages/core/src/reference/guidance.ts packages/core/src/mcp/guidance.ts packages/core/src/instruction-context.ts packages/core/src/session/context-entry.ts packages/core/src/system-context/builtins.ts packages/core/test/system-context/index.test.ts packages/core/test/instruction-context.test.ts packages/core/test/session-runner.test.ts
• Pre-push hook: bun turbo typecheck