Task tool description ignores session permissions

Description

The Task tool's model-facing subagent list ignores session permissions. describeTask filters with agent.permission, while execution and the adjacent Code Mode description merge agent and session rules. A session-denied target is still advertised; a session-allowed target can remain absent.

This is a description mismatch, not a claim that execution bypasses permissions. #41100 included this fix alongside other changes but was automatically closed without merging. PR #48107 fixes session permission merging only.

Scope relative to the related issues

• #35238 concerns the V2 subagent tool in packages/core, including runtime enforcement. This issue concerns the legacy Task description in packages/opencode/src/tool/registry.ts.
• #45078 concerns copying superseded parent denies into a child's permissions in deriveSubagentSessionPermission. This reproduction creates no child session; the mismatch already exists in registry.tools() output.
• #39086 reports the entire task tool missing. Here task is present, but its available-agent list includes session-denied targets or omits session-allowed targets.

The fix only supplies Permission.merge(agent.permission, session.permission ?? []) to describeTask. It changes neither rule evaluation semantics nor execution checks, child-session inheritance, or whole-tool visibility. Four session-override regression cases fail on the unmodified base and pass with #48107; omitted/empty session permissions retain their behavior.

OpenCode version

dev at 830d5eb5354874105cc31599635a80c1662609e8

Steps to reproduce

1. Call ToolRegistry.Service.tools with an agent whose Task rules deny * and then allow explore.
2. Pass permission: [{ permission: "task", pattern: "explore", action: "deny" }].
3. Inspect the returned task.description: its available-agent list still includes explore; it should omit it.
4. Conversely, a session allow or ask for general should make it visible despite the agent's wildcard deny, but currently does not.

Code: https://github.com/anomalyco/opencode/blob/830d5eb5354874105cc31599635a80c1662609e8/packages/opencode/src/tool/registry.ts#L265-L269