feat(core): discover AGENTS.md up to the home directory

[kitlangton]

What

Extend ambient AGENTS.md discovery from the project-root boundary to the user's home directory when the resolved Location is inside home. Locations outside home keep the existing project-root boundary.

Why

This enables shared per-directory conventions such as ~/code/AGENTS.md to apply across every project below that directory. Claude Code and pi similarly walk beyond the repository boundary to the filesystem root, while grok and Codex stop at the git root; stopping at home provides shared user conventions without consulting ancestors above projects in temporary directories, CI scratch space, or shared mounts.

How

• Resolve the Global service's home through the same filesystem canonicalization used for the Location and project root.
• Select home as the upward stop with one containment check when the start directory is inside home; otherwise retain the project root.
• Derive both discovery and file-watch candidates from that selected stop, preserving global-first and nearest-to-farthest ordering.
• Keep the existing project/trust gating unchanged.
• Update the instructions and V1 migration docs with the new boundary.

Testing

• bun run test instruction-discovery.test.ts from packages/core: 9 passed
• bunx tsgo --noEmit from packages/core: passed
• bun run test from packages/core: 1,636 passed, 16 skipped
• Push hook workspace typecheck: 33 tasks passed

[kitlangton]

Implementation and verification are complete: focused instruction discovery tests, core typecheck, full core suite, and workspace push-hook typecheck all pass.