feat(canon): §2.G strip UPGRADE_BASE_VERSION entirely (plausible verified dynamic-base green)

Gate satisfied — live: with the pin removed, plausible's upgrade tier resolves base 3.0.1+v2.0.0 via
the same-version step-back (canonical 3.1.0 == head 3.1.0 → newest-older = 3.0.1, NOT the broken
3.0.0) and passes install+upgrade green (level 5/5). The pin is redundant, so removed everywhere:
- meta.py KEYS entry (RecipeMeta field auto-drops; 15→14 keys).
- run_recipe_ci.resolve_upgrade_base override branch + docstrings.
- tests/unit/test_meta.py (count 15→14, dropped None-assert), test_upgrade_base.py (override test).
- docs/recipe-customization.md (regenerated table + mentions), docs/testing.md.
- tests/plausible/recipe_meta.py (pin removed), tests/bluesky-pds (re-enable note → dynamic base).
294 unit tests pass; lint clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

runner/harness/meta.py

@@ -85,12 +85,6 @@ KEYS: tuple[Key, ...] = (
         "Callable `(ctx) -> [probe, ...]` returning extra readiness probes, run after install AND after upgrade: HTTP `{host, path, ok}` or TCP `{tcp_host, tcp_port, stable}`.",
         hook_params=("ctx",),
     ),
-    Key(
-        "UPGRADE_BASE_VERSION",
-        "str",
-        None,
-        "Optional explicit override pinning the upgrade tier's base to an exact published tag (rare; for a PR that adds a version *above* the newest tag). When unset (the norm) the base is resolved DYNAMICALLY (phase prevb): last-green (warm canonical) → target-branch (`main`) tip → else skip. See `run_recipe_ci.resolve_upgrade_base` + DECISIONS.",
-    ),
     Key(
         "BACKUP_VERIFY",
         "hook",