refactor(lib)!: lift stack graph algorithms onto StackMetadata

[lettertwo]

This PR is the refactor that makes room for gh stack (github/gh-stack) as a second stack
provider alongside Graphite. No provider yet, and no CLI change.

git-workon-lib/src/stack/metadata.rs is new: a StackMetadata struct plus enumerate, current,
and changeset_walk. graphite.rs shrinks to read_metadata. Stack gains number: Option<u64>,
keyed by every member branch rather than by the stack root.

Only enumerate takes a &Repository, so only enumerate can prune ghost branches. current and
changeset_walk must not prune, since routing needs a deleted stack node visible to tell it from a
typo, and changeset assembly walks through a ghost to reach live descendants.

refactor(lib)! because the workon library surface moves.

Design notes in docs/adr/028-provider-neutral-stack-metadata.md, section 1.

Testing: zero test edits. The existing Graphite suites are the safety net. 585 tests, clippy clean.

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