[OddballGreg]

Issue for this PR

Refs #5148

Type of change

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

What does this PR do?

It lets a plugin short-circuit a tool call from tool.execute.before by supplying the result itself.

tool.execute.before currently only exposes { args }, so a plugin can rewrite arguments but cannot prevent the tool from running. In SessionTools.resolve the runtime always calls the tool after the hook (the native path around L92 and the MCP path around L135). tool.execute.after can rewrite the output, but only after the work already happened, and permission.ask can deny a call but returns an error rather than a usable result.

This adds an optional result?: string to the before-hook output. If a plugin sets it, the tool is not executed and that string is returned as the tool output instead; tool.execute.after still runs on the substituted result. It is wired through both the native and MCP tool paths. Hooks that do not set result behave exactly as before, so it is backwards compatible.

The motivating case is a cache/memory plugin that already holds the answer for a read/grep and wants to return it instead of doing the read. This is the minimal "early termination" slice of the broader middleware pipeline discussed in #5148 (a prior attempt, #18689, implemented a larger version but was auto-closed before review). The Hooks interface is hand-written in packages/plugin, so no SDK regeneration applies.

How did you verify your code works?

bun run typecheck passes for both the opencode and plugin packages (no errors in the changed files), prettier --check is clean, and oxlint reports no errors on the two files. I did not add an automated test: the SessionTools.resolve plugin-hook path has no existing unit-test harness and a full session/plugin integration test would be disproportionate to the change, so I verified by typechecking and tracing both branches. Happy to add coverage if you can point me at the right harness.

Screenshots / recordings

Not a UI change.

Checklist

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

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