feat: configurable plans directory and opt-out for plugin dependency installs

[log0u7]

Issue for this PR

Closes #46189

Type of change

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

What does this PR do?

Opening opencode in a git project and using plan mode creates <project>/.opencode/plans/. On the next start, that directory is picked up as a config directory and gets an automatic @opencode-ai/plugin dependency install, plus generated package manifest, lockfile and .gitignore files. Every project plan mode was used in ends up with its own copy, and ~/.config/opencode gets one too when it is a git repo. There is currently no way to opt out (details and code refs in #46189).

This PR adds two opt-outs, both default-preserving:

• plans_directory config key (supports ~): plan files are written to the configured directory instead of the worktree/global defaults, e.g. the global data dir, so project trees stay untouched. The plan agent's built-in edit/external_directory allows were extended to the configured directory so plan mode keeps working.
• OPENCODE_DISABLE_PLUGIN_DEPS=1 env flag: skips the per-directory dependency install and the generated files. Config discovery (agents, commands, plugins) is unaffected.

Default behavior is unchanged when neither is set.

How did you verify your code works?

• New unit tests for Session.plan(): default paths (git / non-git), plans_directory override (absolute, ~, bare ~, relative), and a config-loader test asserting nothing is written into a config dir when the flag is set
• Full bun run typecheck: 30/30 turbo tasks pass on bun 1.3.14 (the pinned version)
• Manual e2e with the fork build in a throwaway git repo: plan file lands in the configured directory, no .opencode directory is created in the project

Screenshots / recordings

Not a UI change.

Checklist

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

[log0u7]

For anyone landing here from the issue: ways to keep this clean today, before and after this PR.

On released versions (no PR needed):

1. Git-wise you are already safe: opencode writes a .gitignore inside .opencode covering node_modules and the generated manifest files, so nothing reaches your commits. The clutter is filesystem-only.
2. Shared .opencode symlink: make <project>/.opencode a symlink to one shared directory (e.g. ~/.local/share/opencode/workspace). The dependency install happens once, plan files land in a single place, and per-project agents/commands keep working. One caveat: an opencode.json inside the shared directory would apply to every project linked to it. Add .opencode to ~/.config/git/ignore so no repo ever tracks the symlink.
3. OPENCODE_DISABLE_PROJECT_CONFIG=1 stops the per-project dependency installs, but also disables per-project AGENTS.md and config discovery. Only viable if you keep everything global.
4. Periodic cleanup of .opencode works, but opencode reinstalls on the next open, so it is a treadmill.

With this PR:

Options 1-4 become unnecessary. Set plans_directory (e.g. ~/.local/share/opencode/plans) and/or OPENCODE_DISABLE_PLUGIN_DEPS=1 in your environment, and project directories stay completely untouched.