[RealZST]

Problem

After upgrading to v1.15.1, users who had installed skills with a quoted frontmatter name (name: "imagegen") from the marketplace saw them listed twice: "imagegen" and imagegen.

#173 changed how the name is read, which changes the id derived from it, so the next scan reports the same file under a new id. The old row was a marketplace install with install_meta whose file was still on disk, and the stale-row rules in store/sync.rs treated that as a momentary scan gap and kept it forever. Only rows with install_meta were affected; quoted skills that came with an agent or were cloned by hand were pruned and replaced normally.

Fix

• A file the scan now reports for the same agent under another id counts as reassigned, not as a scan gap: the old row is pruned. Matching is on kind, agent and path, and only for skills and plugins, since MCP/hook rows share their config file and a skill in a shared directory has one row per agent.
• The pruned row's install_meta moves to the row that took the file over, unless that row already has its own, so provenance and update checks survive.
• The two copies of the stale-prune loop (full sync and per-agent sync) are replaced by one shared helper; behaviour on every other path is unchanged.

Verification

• New tests cover the reassignment, the no-overwrite rule for existing install_meta, the store-held disabled exemption under reassignment, and that a shared-directory skill is not treated as reassigned (the other agent's provenance stays put).
• Released v1.15.1 binary vs this build against a copy of a real 1015-row database: the old binary keeps the 5 quoted rows; this build removes exactly those 5, carries marketplace provenance to the two rows that lacked it, leaves rows that already had their own untouched, and a second run changes nothing.
• cargo test --workspace, clippy and cargo fmt --check clean.

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