Tag management: rename and merge tags via /tags UI

[unforced]

Why

Today /tags lists tags with counts but offers no way to act on them. Renaming a tag across a vault (e.g., #todo → #task) requires editing every note by hand. Merging two tags into one is the same pain doubled.

Vault already exposes update-tag as an MCP/HTTP tool. This issue is about surfacing it in Lens.

Scope

• Rename: on each /tags row, a "Rename" action. Modal prompt: new name (autocompletes against existing tags; warns if target exists and offers to merge instead). Confirm → call vault's update-tag with rename semantics.
• Merge: multi-select on /tags rows, "Merge into…" action picks the survivor tag. Confirm → vault retags all notes carrying the merged tags to the survivor; deletes the now-empty source tags.
• Both operations: optimistic UI update, rollback on vault error, toast on success ("Renamed X → Y across N notes").
• Sync queue: rename/merge are online-only for v0.3 (require a live vault call). Don't enqueue offline; show a clear "offline — reconnect to manage tags" state.

Edge cases

• Renaming a tag to itself: no-op, UI should prevent it
• Renaming to a tag that exists: surface the merge option, don't silently merge
• Merge with more than 5 source tags: show a confirmation that names each one
• If vault errors mid-merge (partial retag): surface clearly with recovery guidance

Tests

• Rename happy path (mocked vault)
• Rename-to-existing triggers merge prompt
• Merge happy path
• Error rollback

Out of scope

• Tag hierarchy (vault-level question, separate track)
• Bulk tag operations on note selections (different flow — "add/remove tag from these 10 notes")
• Tag descriptions / schemas (future)

Priority

High. One of the top quality-of-life features for anyone who's used their vault a few weeks.