fix: display plan content in plan_exit dialog & bind variant list keybind

[MrRealORG]

Summary

Fixes #512 and improves #521.

1. Display plan content when plan_exit is called (#512)

Problem: After the plan agent finishes writing a plan and calls `plan_exit`, the question dialog only showed the plan file path (e.g., `.mimocode/plans/1749-xxx.md`) — the actual plan content was never displayed. Users had to manually find and open the plan file to see what was planned. The write tool call that created the plan file was collapsed in the message history.

Fix:

• `packages/opencode/src/tool/plan.ts`: The `plan_exit` tool now reads the plan file content via `fs.readFileSync` and includes it in the question`s `params.planContent`.
• `packages/opencode/src/cli/cmd/tui/routes/session/question.tsx`: The question rendering component now checks for `plan_exit` questions with `planContent` params and renders the plan text in a bordered box below the question text. Users see the full plan content while deciding whether to switch to the build agent.

2. Bind variant list keybind & add slash command alias (#521)

Problem: The `/variants` command (to pick model thinking intensity) had its keybind set to `"none"` (unbound). Users could only access it by typing `/variants` — there was no keyboard shortcut. Additionally, `/variant` (singular) was not recognized as an alias.

Fix:

• `packages/opencode/src/config/keybinds.ts`: Changed `variant_list` keybind from `"none"` to `"ctrl+shift+t". This complements the existing \ctrl+t` (cycle) shortcut.
• `packages/opencode/src/cli/cmd/tui/app.tsx`: Added `"variant"` as a slash command alias for `"variants"`.