docs(roadmap): add isolated mount source branch policy

[donbeave]

Summary

This docs-only PR adds a roadmap item for source branch policy on isolated mounts: operators should be able to choose whether worktree and clone mounts start from host HEAD, the host's current branch, the repository default branch, a named branch, or an ask-on-launch dialog. The roadmap item also captures the TUI direction: replace the current one-key isolation cycling with a mount settings dialog for source folder, container destination, access mode, isolation, and mode-specific source branch settings.

What ships

• A new roadmap item, Source Branch Policy for Isolated Mounts, covering the operator problem, product positioning, policy vocabulary, runtime semantics, CLI shape, schema implications, TUI redesign, implementation scope, deferred work, and open questions.
• Concrete TUI mockups for the workspace editor mount row, mount settings dialog, container-destination text input, named-branch picker, and launch-time branch prompt, including the product promise that the host checkout stays unchanged.
• Roadmap sidebar and overview wiring so the item is discoverable under Agent Orchestrator Research phase 1 and from the roadmap overview.

What this addresses

• The existing per-mount isolation story does not say how operators intentionally choose the branch that a private worktree or clone starts from.
• The current console interaction model for mount isolation is too shallow for mode-specific settings; this roadmap item records the need for a richer isolation settings dialog before implementation starts.

Not included

• No runtime, schema, CLI, or TUI implementation ships in this PR.
• No user-facing Workspaces or Mounts guide changes ship yet; those belong with the implementation PR that makes the behavior real.

Verify locally

Checkout

Paste this first to bypass the tirith paste scanner for the rest of the session:

export TIRITH=0

Then paste the checkout block:

export JACKIN_PR_TEST_DIR="$HOME/Projects/jackin-project/test/pr-580"
mkdir -p "$JACKIN_PR_TEST_DIR"
cd "$JACKIN_PR_TEST_DIR"

if [ ! -d jackin/.git ]; then
  git clone https://github.com/jackin-project/jackin.git
fi

cd jackin
git fetch -f origin pull/580/head:refs/remotes/origin/pr-580-head
git checkout -B pr-580 refs/remotes/origin/pr-580-head
cargo xtask pr prepare 580
cd "$JACKIN_PR_TEST_DIR/jackin"
source "$JACKIN_PR_TEST_DIR/env.sh"
which jackin

Docs checks

(
  cd docs
  bun install --frozen-lockfile
  bun run build
  bun run check:repo-links
  bunx tsc --noEmit
  bun test
)

Documentation

(
  cd docs
  bun install --frozen-lockfile
  bun run dev
)

Vite serves at http://localhost:3000/. Pages to walk:

http://localhost:3000/reference/roadmap/source-branch-policy-isolated-mounts/
NEW page under Behind jackin' / Internals → Roadmap → Agent Orchestrator Research → Fleet phase 1. Confirm the policy vocabulary, product positioning, and TUI mockups describe the source-branch flow clearly.

http://localhost:3000/reference/roadmap/
UPDATED overview. Confirm Source branch policy for isolated mounts appears under Planned → Operator surface and fleet operations.

Migration notes

None. This PR is docs-only.