feat(opencode): filter instruction files by reader audience

[iceteaSA]

Issue for this PR

Closes #38961

Related: #4096 (context control for subagents — shapes context volume from the dispatch side; this shapes ownership from the file side).

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Lets an instruction file declare who it is for, so a nested AGENTS.md written for the orchestrator does not get delivered to every subagent:

---
opencode:
  audience:
    - role: main
    - role: subagent
      agent: "review*"
---

role is main / subagent / all; agent is a glob. Entries OR together, keys within one entry AND together. A file with no opencode: metadata is included everywhere, byte-identical to today — no behaviour change unless you opt in.

Three decisions worth calling out, because each has a wrong-looking-but-tempting alternative:

Reader role is structural, not configured. It comes from session.parentID == null, not from the agent's mode. A mode: "primary" agent dispatched as a subagent is a subagent — deriving role from config would get that backwards, and the whole point is to reason about the delivery, not the label.

An unknown key under opencode: is fatal and names the file. audiance: is valid YAML, so a permissive schema cannot distinguish a typo from an absent directive — and failing open silently reinstates exactly the delivery the directive was written to prevent. opencode: {} is treated as absence and included, since an empty map asserts nothing.

Filtering happens on every delivery path, not just the obvious one. buildSystem() is the path people think of; two others leak without it, and both were found by inspection rather than by a failing test:

• Instruction.resolve() — nested AGENTS.md surfaced through the read tool. Needed a reader on Tool.Context, supplied by the production constructors and failing closed when absent.
• createUserMessage's file: attachment path — hardcoded role: "main", so a subagent leaked main-only instructions whenever a dispatch carried an @file reference.

AudienceError is delivered as a typed failure rather than a defect, so a malformed file surfaces as a normal error at the call site.

Known limitation, documented in rules.mdx: the V2 SessionCore assembles system context through a separate path (packages/core/src/instruction-context.ts) with no audience filtering. That is bounded today because V2 has no task tool, so every V2 session is structurally main — but it stops being bounded the moment V2 gains dispatch. packages/core cannot import from packages/opencode, so sharing the filter means moving it into core; that is a larger change than this PR should carry.

How did you verify your code works?

• New packages/opencode/test/session/instruction-audience.test.ts covers parsing, role/agent matching, wildcard semantics, OR/AND composition, directive stripping, and malformed-metadata failures.
• Two-way mutation-tested intersection test in instruction.test.ts: neutering the audience filter leaks wrong-audience project files; neutering the origin filter leaks config files into project scope. Each failure names which filter died.
• Red-first proof for both leak paths — subagents receiving nested main-only instructions through resolve(), and through an @file attachment — each failing before the fix and passing after.
• Whole-branch mutation check: making the filter a no-op turns 11 tests red across audience exclusion, frontmatter stripping, and typed-failure delivery; restoring returns 99 pass / 0 fail.
• bun test in packages/opencode: 3243 pass / 0 fail (dev baseline 3207). bun typecheck clean in packages/opencode and packages/core.
• Docs updated in packages/web/src/content/docs/rules.mdx, including the V2 limitation above.

Screenshots / recordings

Not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR