refactor(ui)!: remove the classic v2 shell — v3 is the only UI (R3a) (#871)

* refactor(ui)!: remove the classic v2 shell — v3 is the only UI (R3a)

Deletes `static/index.html`, the `/v2` route, and the `FEEDBACK_UI` v2/legacy
opt-out. `/` and `/v3` both serve `static/v3/index.html`, which has been the
default since 0.3.0.

This is step 0 of the core-frontend ES-module migration (R3a). Both shells load
the same `static/app.js`, so every later step of that migration — exposing the
window contract, the `defer` ordering fix, the `type="module"` flips — would
otherwise have to be made and verified twice. Removing the fallback now halves
that surface before any of it is touched.

Incidentally fixes a latent bug in `index()`: its guard read
`if getenv_compat("FEEDBACK_UI") or getenv_compat("FEEDBACK_UI") in ("v2", "legacy")`,
whose left operand is truthy for *any* non-empty value — so `FEEDBACK_UI=v3`
actually served the **v2** shell.

- `static/tailwind.min.css` regenerated: the content globs scanned the deleted
  file, so v2-only utility classes are now purged (CI's tailwind-fresh job
  rebuilds and diffs it).
- Constitution amended to 1.3.0 — Principle II's frontend file list now names
  `static/v3/index.html`.
- Tests: 4 suites read the v2 shell (3 via a constructed `path.join` that a
  literal grep misses). Their v2 halves are paired duplicates of v3 tests that
  stay, so they are dropped; `alpha_warning_banner` and the capability-registry
  script-order test retarget to `static/v3/index.html`.

BREAKING CHANGE: `FEEDBACK_UI=v2` / `=legacy` and the `/v2` route are gone.
Unset the variable and use `/`. No chart, settings, or plugin data changes, and
no plugin API changes — v3 reuses the same engine.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* docs: drop the stale '/ is v2' plugin-verification guidance (CodeRabbit)

The v3-only rewrite updated the intro paragraphs but left three lines that
still instructed plugin authors to verify in 'both / (v2) and /v3' — now the
same shell. Historical 'in v2 it was X' contrasts are kept: they still orient
authors whose plugins also ship to users on older cores.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_env_compat.py

@@ -4,22 +4,22 @@ from env_compat import getenv_compat, env_flag_compat
 
 
 def test_canonical_wins_over_legacy(monkeypatch):
-    monkeypatch.setenv("FEEDBACK_UI", "v3")
-    monkeypatch.setenv("SLOPSMITH_UI", "v2")
-    assert getenv_compat("FEEDBACK_UI") == "v3"
+    monkeypatch.setenv("FEEDBACK_DEMO_MODE", "canonical")
+    monkeypatch.setenv("SLOPSMITH_DEMO_MODE", "legacy")
+    assert getenv_compat("FEEDBACK_DEMO_MODE") == "canonical"
 
 
 def test_legacy_fallback_when_canonical_unset(monkeypatch):
-    monkeypatch.delenv("FEEDBACK_UI", raising=False)
-    monkeypatch.setenv("SLOPSMITH_UI", "v2")
-    assert getenv_compat("FEEDBACK_UI") == "v2"
+    monkeypatch.delenv("FEEDBACK_DEMO_MODE", raising=False)
+    monkeypatch.setenv("SLOPSMITH_DEMO_MODE", "legacy")
+    assert getenv_compat("FEEDBACK_DEMO_MODE") == "legacy"
 
 
 def test_default_when_neither_set(monkeypatch):
-    monkeypatch.delenv("FEEDBACK_UI", raising=False)
-    monkeypatch.delenv("SLOPSMITH_UI", raising=False)
-    assert getenv_compat("FEEDBACK_UI", "default") == "default"
-    assert getenv_compat("FEEDBACK_UI") is None
+    monkeypatch.delenv("FEEDBACK_DEMO_MODE", raising=False)
+    monkeypatch.delenv("SLOPSMITH_DEMO_MODE", raising=False)
+    assert getenv_compat("FEEDBACK_DEMO_MODE", "default") == "default"
+    assert getenv_compat("FEEDBACK_DEMO_MODE") is None
 
 
 def test_non_feedback_names_have_no_fallback(monkeypatch):
@@ -31,9 +31,9 @@ def test_non_feedback_names_have_no_fallback(monkeypatch):
 
 def test_empty_canonical_is_respected_not_overridden(monkeypatch):
     # An explicitly-empty canonical value is still "set" and wins.
-    monkeypatch.setenv("FEEDBACK_UI", "")
-    monkeypatch.setenv("SLOPSMITH_UI", "v2")
-    assert getenv_compat("FEEDBACK_UI") == ""
+    monkeypatch.setenv("FEEDBACK_DEMO_MODE", "")
+    monkeypatch.setenv("SLOPSMITH_DEMO_MODE", "legacy")
+    assert getenv_compat("FEEDBACK_DEMO_MODE") == ""
 
 
 def test_flag_parses_true_values(monkeypatch):