Source Branch Policy for Isolated Mounts

Status: Open — design only, nothing implemented; MountConfig has no source-branch field and worktree/clone materialization always starts from the host repo's current HEAD

Shipped baseline

Per-mount shared | worktree | clone isolation is shipped (MountConfig in crates/jackin-config/src/schema.rs), but the isolation type carries no source-branch policy. Worktree materialization resolves the base commit with a plain git -C <src> rev-parse HEAD (crates/jackin-isolation/src/materialize.rs), so every isolated checkout inherits whatever the host repo happens to have checked out at launch time, including detached HEAD. There is no --mount-source-branch CLI flag, no TUI control for it, and no workspace-schema field for it today.

Remaining work

1. Workspace schema. Add source_branch_policy (+ source_branch for the named case) to MountConfig in crates/jackin-config/src/schema.rs, with a versioned migration in crates/jackin-config/src/migrations.rs (bump CURRENT_WORKSPACE_VERSION, add a migration fixture, re-bake existing after.toml fixtures, add a Schema Versions timeline entry).
2. Runtime resolution. In crates/jackin-isolation/src/materialize.rs, resolve the policy (host_head / current_branch / default / named / ask) to a concrete ref before worktree add or clone, without ever mutating the host checkout (no implicit switch/checkout/pull/fetch).
3. CLI. Add --mount-source-branch <dst>=<policy>[:<branch>] to jackin workspace create/edit in crates/jackin/src/cli/workspace.rs, validated the same way --mount-isolation is today.
4. Console editor. Replace the current isolation row-cycling in crates/jackin/src/console/tui.rs with a mount settings dialog (source folder, container destination, readonly, isolation, mode-gated source-branch group), plus a named-branch picker and an ask-on-launch prompt reusing the existing launch dialog surface (crates/jackin-runtime/src/runtime/progress.rs).
5. Tests. Parsing, migration, policy resolution, worktree/clone materialization, invalid combinations, and TUI state transitions.
6. Docs. Workspaces guide, Mounts guide, workspace command reference, configuration reference, schema versions — once the feature ships.

Out of scope (for this item)

• Implicit fetch/pull of branch lists — local refs only.
• Per-role source-branch defaults.
• Grouped multi-mount ask dialog — one mount at a time.
• Source-branch policy for shared, tmpfs, or ephemeral modes.
• Automatic PR-branch discovery as a policy option.

Related work

• Research: Source Branch Policy for Isolated Mounts — full problem framing, product positioning, TUI mockups, data model, and open design questions.
• Ephemeral mount modes — another sign that isolation is becoming a configurable mount strategy, not a single enum.
• Session keep and resume — resume becomes more predictable when every isolated checkout records exactly which branch/commit it started from.
• Launch Progress TUI — provides the rich launch dialog surface for Ask on launch.
• Task source abstraction — future PR/task flows can reuse this policy once a task carries its own branch source.