Add plants:sync_common_names for the ECHOcommunity plant migration

[npflood]

Phase 1 of the plant-data ownership migration. Independent of #117, #118 and
#119 — merge in any order.

Syncs common names from ECHOcommunity and sets the primary flag per language.

bin/rails plants:sync_common_names[tmp/common_names.json]              # dry run
APPLY=true bin/rails plants:sync_common_names[tmp/common_names.json]   # write

Why the primary flag is the point

Plant#primary_common_name_for_locale only uses a language when a primary is
set in it, otherwise falling back to English. So a language holding names but no
primary displays English, and the translated names sit unused in the same
table.

That was the state before this: Thai had 1 primary across 186 names. A Thai
reader saw English names for essentially every plant, with Thai names present
and unreachable.

Which name becomes primary

Decided upstream in the migration workspace, where the rules and their reasoning
live together, so this stays a simple apply step. In order:

1. only one name in that language → it is primary
2. a name equals the ECHOcommunity title → that one
3. a name equals a fragment of the title → that one, first fragment wins

Rule 3 does the heavy lifting because ECHOcommunity titles are composites —
"Coriander / Cilantro", "Caigua (achogcha, archucha, caihua…)" — so exact
matching alone resolved almost nothing. Where none of those apply it falls back
to word overlap, then to the lowest-id name, which at least yields a name in
the reader's language rather than English.

Behaviour

Additive for names: missing ones are created, existing ones matched
case-insensitively on (name, language) and never duplicated. Deleting a name
ECHOcommunity no longer lists is an editorial act, not a migration one. The
primary flag is synced in both directions, since exactly one name per language
should carry it.

Rehearsed against staging

57 names created, 1,741 already present, 546 primary flags set, 16 cleared, no
failures. Coverage afterwards:

language	before	after
Thai	1 / 186	207 / 245
Spanish	53 / 228	107 / 243
French	40 / 105	74 / 108
Chinese	—	36 / 46

• 6 new specs
• Full suite: 2,310 examples, 0 failures
• Whole-repo rubocop clean