feat(cli): register new branches in the canonical gh-stack file

[lettertwo]

This PR makes workon new register the branch it creates in the canonical gh-stack file.

Upstream's cmd/add.go unconditionally runs git.CheckoutBranch(branchName), and git's
one-worktree-per-branch rule blocks both checkouts it would need: the new branch is already live in
the new worktree, and the base branch is live wherever it was created. workon writes the file
directly instead.

gh_stack::register_branch appends branch to the stack that currently ends at base_branch, with
base set to repo.merge_base(base tip, branch tip) so it stays correct if the base moved. The
document round-trips through serde_json::Value rather than a typed struct, so id, pullRequest,
and any field a future gh-stack adds survive on untouched entries. The write is temp-and-rename
against canonical itself, never a worktree's symlinked path, since renaming onto a symlink replaces
the link with a real file.

The read happens under the lock rather than before it. Once the lock is held no lock-respecting
writer can be mid-write, so a pre-lock read is the only way plan_registered_doc could receive a
truncated prefix.

workon.stackAutoTrack is the new config key covering both providers. workon.gtAutoTrack stays as
a deprecated alias, read only when stackAutoTrack is unset. Registration failures stay non-fatal
warnings, matching the Graphite path.

Testing: 647 tests, clippy clean.

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