fix: store persistent state under XDG state directory

[mikemikimike]

Problem

Hunk currently stores its persistent state file at $XDG_CONFIG_HOME/hunk/state.json, alongside the user-editable configuration at $XDG_CONFIG_HOME/hunk/config.toml.

This mixes two different kinds of data defined by the XDG Base Directory Specification: XDG_CONFIG_HOME is intended for user-editable configuration, while XDG_STATE_HOME is intended for data that persists across restarts but is not important or portable enough to belong in XDG_DATA_HOME. The state file and the user-scoped directory used for globally installed extensions belong in the latter category.

Changes

• Resolve state.json from $XDG_STATE_HOME/hunk, falling back to ~/.local/state/hunk when XDG_STATE_HOME is unset.
• Keep config.toml under $XDG_CONFIG_HOME/hunk, with the existing ~/.config/hunk fallback.
• Move global extension discovery and managed extension installs to the same XDG state directory.
• Add regression coverage for explicit XDG directories and state-directory fallbacks.

Validation

• bun test src/core/run/paths.test.ts src/extensions/discovery.test.ts src/extensions/startup.test.ts src/extensions/manage/install.test.ts
• bun run typecheck
• bun run lint
• bun run deps:check
• Changed files pass the formatter check.
• git diff --check

The repository-wide format check reports pre-existing failures elsewhere in the checkout; the changed files pass the formatter check.

Compatibility / Known limitations

This changes the default location of persistent state and globally managed extensions on XDG-compatible systems. User-editable configuration remains at its existing location. Existing state or extensions under the old configuration directory are not migrated automatically.

Fixes #821

[greptile-apps]

PR author is not in the allowed authors list.

[vercel]

@mikemikimike is attempting to deploy a commit to the Modem Team on Vercel.

A member of the Team first needs to authorize it.

[benvinegar]

Can you fix the PR description? It's poorly formatted. What problem are you solving, etc? (I can guess, but ...)