Step 1 of the snippet-merge milestone (#231). A snippet that unifies N
found one-offs carries N locations (one per call site), so `location`
becomes a list. Ships on the body-convention — no migration, swappable to
the deferred `data` JSONB later (as #227 decision 5 anticipated).
- compose_body: renders `**Location:**` for a single location, a
`**Locations:**` bullet list for several. Accepts a `locations` list;
the single repo/path/symbol params remain as a one-location shorthand
(create path + existing callers/tests unchanged).
- parse_snippet_fields: reads BOTH the new `**Locations:**` list block AND
the legacy single `**Location:**` line (tolerant, never raises); returns
a `locations` list and mirrors the first into repo/path/symbol for
back-compat (rule #33).
- update_snippet: gains a `locations` param — replaces the whole set; else
a legacy single triple overlays onto the first location; else kept.
- Tests: multi-location round-trip, singular-vs-plural label, legacy
single-line parse, normalize dedup/empty.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Pa2EsuB54BuWQ8GfJq9c7t
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