proposal: extend AGENTS.md upward discovery beyond the project root, stopping at $HOME #41633

Current behavior

InstructionDiscovery walks AGENTS.md from the session directory up to the canonical project root and stops (plus the global ~/.config/opencode/AGENTS.md). Directories between the project root and the user's home are never consulted.

Proposal

Continue the upward walk past the project root, stopping at $HOME (inclusive). No other changes: same nearest-to-farthest stacking, same admission/rendering, same global file.

Why

It enables the in-between zone that neither per-project nor global config can express today: a ~/code/AGENTS.md applying shared conventions to every project under ~/code/ ("always bun, never npm", house lint rules, etc.). Users arriving from Claude Code and pi already expect ancestor context files above the repo to apply — both walk to the filesystem root.

Prior art

Stopping at $HOME rather than / keeps shared/world-writable ancestors (/tmp, mounts, CI scratch dirs) out of sessions rooted there, at the cost of one boundary rule.

Mechanics

After the instruction-discovery refactor (#41629), discovery reads a static candidate list (one AGENTS.md path per ancestor directory) with one file-watch per candidate. This proposal is: extend the chain's stop from project root to $HOME — a different stop argument and a slightly longer watch list. Small diff, fully testable with the existing harness.

Non-goals

Downward/nested discovery (VS Code useNestedAgentsMdFiles-style) is separate — the lazy read-tool path (SessionInstructions) already covers subtree context during a session.