docs: document the gh-stack stack provider

[lettertwo]

Documentation only, no code. Split out so the code changes stay reviewable on their diffs alone.

docs/adr/028-provider-neutral-stack-metadata.md is new and carries the decisions the gh-stack work
implements: the shared StackMetadata boundary, why reads never shell out, why Graphite wins
detection, why workon new writes the gh-stack file itself, and the canonical-file-plus-symlinks
layout. It quotes the two upstream Go mechanics the symlink approach depends on with file
references, so the claims can be checked against github/gh-stack.

docs/recipes/stacked-diffs.md gains the gh-stack workflow alongside the Graphite one.
docs/rfc/stacked-diffs.md, README.md, and docs/INDEX.md pick up the second provider. ADR-023
and ADR-024 get short amendments where provider-neutrality changed what they had described as
Graphite-specific.

ADR-028 is the one worth reading closely. The rest is cross-referencing and index upkeep.

Testing: 647 tests, clippy clean.

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