feat(core): deliver CodeMode catalog through instructions - #38003

Moves the dynamic CodeMode tool catalog out of the execute tool description and into a durable Instruction source, per the CodeMode catalog design (#36196).

Changes

• Stable execute description: CodeMode.description in @opencode-ai/codemode is now invariant guidance (runtime, tools.* paths, search, Promise.all). The provider tool definition no longer changes when the catalog changes.
• New core CodeMode.Service (packages/core/src/codemode.ts): owns CodeMode tool registrations (scoped, latest-active-wins) and materialization. ToolRegistry.register delegates codemode: true registrations to it; the registry keeps only native/direct tools.
• New CodeModeInstructions source (core/codemode): renders the permission-filtered catalog as an agent-scoped Instruction, with initial, changed ("catalog has changed... supersedes the previous"), and removed renderings. Wired into session context loading alongside the other instruction sources.
• ExecuteTool now exposes instructions(registrations) separately from tool creation, so the catalog renders without constructing a discovery runtime inside the tool description.

Tests

• CodeMode.Service registration + materialization
• CodeModeInstructions initial/changed/removed rendering
• Codemode package: stable description stays independent of the catalog
• Updated MCP + tool-registry tests to assert the catalog no longer rides in execute.description

bun typecheck clean in packages/codemode and packages/core. Full suites pass in both (one pre-existing unrelated failure in test/config/plugin.test.ts, fails identically without these changes).