V2: add tool registration to Promise plugins

[opencode-agent]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Implement the V2 architecture for adding tools from plugins and migrating built-in tools toward plugin-backed implementations.

Context from the V2 beta discussion:

• Plugins should be able to add tools.
• Internal tools should move toward the same plugin architecture where practical.
• The read tool should eventually be a plugin and should own progressive AGENTS.md discovery/injection behavior when reading files.

Expected scope:

• Define the plugin API for registering tools.
• Wire plugin-provided tools into the V2 tool registry/materialization path.
• Identify how built-in tools migrate to the same mechanism.
• Include the read-tool/AGENTS.md integration in the design, even if implementation is split.
• Add tests for plugin tool registration and availability.

[opencode-agent]

Tracker update from V2 TODO LIST audit (2026-07-02):

Recent/open plugin-tool architecture work:

• refactor(plugin): move tool implementation to plugin #34665 merged: moved the canonical V2 Effect tool API into @opencode-ai/plugin/v2/effect/tool and kept core re-export compatibility.
• feat(plugin): add tool result content API #34709 open: adds Tool.result({ output, content }), progress/result-content API, and migrates built-ins toward explicit output/content results.
• refactor(core): bridge plugin runtime in app builder #34805 open draft: bridges plugin runtime in the app builder / LayerNode wiring.

Suggested tracker state: keep this issue open until #34709/#34805 land and we verify the remaining plugin/tool architecture gaps (request hooks, resource tools, CodeMode/MCP adapter interactions).

[opencode-agent]

Additional tracker update (2026-07-02):

Related open PR:

• refactor(core): bridge plugin runtime in app builder #34805 — refactor(core): bridge plugin runtime in app builder

This PR is not only a refactor; it affects the runtime seam that plugin/tool architecture depends on. Keep this issue open until #34805 and #34709 settle and the plugin runtime/tool result APIs are verified together.

[opencode-agent]

Update: #34956 (refactor(core): migrate built-in tools to internal plugins) continues the internal-plugin tool migration after #34946. Once it lands, re-check whether the V2 built-in tool/plugin sequencing concerns in this tracker are resolved or should be narrowed to remaining public plugin API details.

[kitlangton]

Most of this tracker has landed via #34619, #34665, #34946, and #34956: Effect plugins can register tools, the canonical tool type is plugin-owned, built-ins use the same registration path, and registration/materialization are tested.

Keeping this open with narrower remaining scope: config-discovered Promise plugins still have no ctx.tool API (packages/plugin/src/v2/promise/context.ts and packages/core/src/plugin/promise.ts). #34709 and #34805 are adjacent work, not blockers for the already-complete Effect-side architecture.