configurable gitbutler storage path via Git config

[Byron]

Fixes #11157 and resolves the issue of Nightly builds putting the database into the future, making it unreadable with the stable build.
As this new default for .git/gitbutler is configurable, the user can force both build types to

Tasks

• review
• test in the real
  • see how one would be able to 'revert' the behaviour by configuring the storage path for the nightly channel
• protection for db clashes when configuration is setup globally (must get more unique DB name then)
• validate CI

Codex Plan

Commit-Ready Plan: Configurable GitButler Storage Path with Channel Isolation

Summary

Change GitButler project storage to be resolved from Git config (gitbutler.storagePath) and default non-stable builds to channel-specific storage paths (.git/gitbutler.<channel>).
This addresses two motivations:

1. Prevent channel interference from incompatible DB schemas/versions.
2. Unblock repos on unsuitable drives (for example NFS/network mounts in #11157) by letting users point storage to a safe local path.

Scope and behavior

1. Add a repository-local config key: gitbutler.storagePath.
2. Resolution rules:
3. If gitbutler.storagePath is absolute: use it directly.
4. If gitbutler.storagePath is relative: resolve against repo.git_dir().
5. If unset:
6. Release builds default to <git_dir>/gitbutler.
7. Non-release builds default to <git_dir>/gitbutler.<channel>.
8. If channel is unavailable, treat as dev and use <git_dir>/gitbutler.dev.
9. No automatic migration/fallback from legacy .git/gitbutler.
10. Do not auto-write default path into .git/config.

Required implementation changes

1. Introduce a shared resolver in but-core (RepositoryExt) so all callers use one implementation.
2. Update but-ctx::Context to store the resolved project-data directory eagerly at creation time.
3. Ensure all Context constructors create/open a real repository before finalizing context so full Git config is available.
4. Route DB initialization in Context through the resolved project-data directory, not a hardcoded gitdir.join("gitbutler").
5. Update legacy project management code (gitbutler-project add/get/delete storage dir operations) to use the same resolver logic.
6. Update documentation/comments that currently imply only .git/gitbutler.

Public API / interface changes

1. New recognized Git config key: gitbutler.storagePath.
2. New RepositoryExt method for resolved storage path.
3. Context::project_data_dir() becomes config-aware instead of hardcoded.
4. Context and thread-safe context equivalents carry the resolved project-data directory as state.

Testing and acceptance criteria

1. Resolver tests:
2. Unset config + release => <git_dir>/gitbutler.
3. Unset config + nightly => <git_dir>/gitbutler.nightly.
4. Unset config + unknown non-release channel => <git_dir>/gitbutler.<channel>.
5. Config absolute path respected exactly.
6. Config relative path resolves against git_dir.
7. Context tests:
8. Creating context with configured path yields matching project_data_dir.
9. DB path uses resolved directory.
10. Thread-safe conversion preserves resolved directory.
11. Legacy project controller tests:
12. Add/get create the resolved storage directory.
13. Delete removes the resolved storage directory.
14. Non-regression:
15. Existing stable default remains .git/gitbutler.
16. No implicit migration/fallback behavior.

Rollout and compatibility notes

1. Nightly/dev users with existing .git/gitbutler data will start isolated storage by default.
2. Users affected by issue Database errors when loading repo from NFS Share #11157 can set gitbutler.storagePath to a local filesystem path suitable for SQLite.
3. Release channel behavior remains unchanged unless override is configured.

Proposed commit message

Subject
ctx/project: make storage path git-configurable and channel-isolated

Body
Resolve GitButler project storage from gitbutler.storagePath and make non-release defaults channel-specific (.git/gitbutler.<channel>) to prevent channel interference from incompatible DB versions.

This also enables moving project storage off unsuitable repository drives (e.g. NFS/network mounts), addressing SQLite lock/rename failures reported in #11157.

Behavior notes: no automatic migration/fallback from legacy .git/gitbutler, and defaults are computed in-memory (not auto-persisted to git config).

Assumptions locked

1. Config key is gitbutler.storagePath.
2. Relative config paths are git_dir-relative.
3. Isolation-only strategy for existing legacy directory.
4. Legacy gitbutler-project storage directory operations are included in this same change.

[vercel]

@Byron is attempting to deploy a commit to the GitButler Team on Vercel.

A member of the Team first needs to authorize it.