feat(cli): link gh-stack worktrees and check them in doctor

[lettertwo]

gh stack writes one JSON file per git dir, which for a linked worktree is
<common-dir>/worktrees/<name>/gh-stack. A stack created in one worktree is invisible from every
other one, which breaks list, routing, and changeset assembly. Upstream's own
skills/gh-stack/SKILL.md says the local file "would be wrong or absent" for worktree workflows.

This PR makes <common-dir>/gh-stack the one canonical store, with link_worktree symlinking
gh-stack and gh-stack.lock into each worktree's admin dir. gh-stack-rebase-state and
gh-stack-modify-state stay per-worktree, since sharing them would make every other worktree
believe it is mid-rebase. migrate_worktree moves a pre-existing real file into canonical with a
backup, and doctor --fix calls it.

doctor gains GhStackWorktreeNotLinked, GhStackDivergentStacks, GhStackNotInitialized,
GhStackExtensionNotFound, GhStackFileUnreadable, GhStackLocked, and BothStackToolsDetected.
The Graphite-specific stack errors become provider-neutral.

docs/adr/028-provider-neutral-stack-metadata.md covers why the symlinks work and why
read_metadata keeps a union fallback for worktrees that drift out of the linked state, under "The
shared canonical file" and the two sections after it. The claims about upstream's Save and Lock
are quoted there with file references.

Acceptance-tested against a copy of this repo's real 33-branch gh-stack file: doctor --fix
migrated it preserving all 33 branches, every base SHA, the trunk, and repository, and
gh stack view --json then parsed the workon-written file from two worktrees.

Testing: 630 tests, clippy clean.

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