feat(snippets): record merge provenance on the survivor

Merge kept the target's fields, unioned locations and tags, and trashed
the sources — recording nothing about what it absorbed. If a variant
handled an edge case the survivor doesn't, that difference left the
visible record entirely; recovering it meant knowing to go digging in
the trash.

The survivor now carries `merged_from`: a "**Merged from:** #2, #3" line
in the body for humans, and the same list in the `data` mirror for
queries, written from one value like every other field (#2087).

It accumulates rather than replaces — a target merged twice keeps both
histories — and skipped sources (cross-owner, per #231) are excluded, so
the record never claims to contain something it never absorbed.

Ordinary edits carry it forward. update_snippet recomposes body and
mirror from scratch, so an omission there would silently erase the
history on the next unrelated edit; that path is pinned by its own test,
including the pre-0070 case where the body line is the only copy.

Surfaced in the snippet detail view as a "Merged from" row — the view
renders parsed fields, not the raw body, so the body line alone would
have been invisible to the operator (rule #27).

Un-merge, the other half of #2087, stays open: restoring a source from
trash still doesn't strip its locations off the survivor, and what
partial un-merge should mean is a design question, not a coding one.
`merged_from` is the record that makes it tractable.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RLwAaV4DQEmVyn496HnEvt

src/scribe/mcp/tools/snippets.py

@@ -243,6 +243,10 @@ async def merge_snippets(target_id: int, source_ids: list[int]) -> dict:
     trash (recoverable). The survivor is re-embedded so recall stops surfacing the
     now-merged duplicates.
 
+    The survivor records what it absorbed as `merged_from` (in its `snippet`
+    fields and as a "Merged from: #ids" line in the body), so a variant that got
+    folded in leaves a trace outside the trash. It accumulates across merges.
+
     Args:
         target_id: The snippet to keep (the canonical record).
         source_ids: Snippet ids to fold into the target and retire. Ids that