Remove the title field — a note is named by its first line

Operator (note 2897): "notes shouldn't have a title field." The concept of a NAME
stays — search results, export filenames and the command palette all need one —
but nothing is typed into it any more. `display_title` is now the first non-empty
line of the body, falling back to the first checklist item.

That fallback is what step 2 bought, and the reason this could not go first: a
checklist had no body to be named from, so the title was its only name. Now every
note has a body, and a note that is only a checklist is named by its first item.

Gone everywhere: the column and note_revisions.title (0026), the field on the
core's Note/NoteCreateInput/NoteRevision and its SQLite columns (user_version 7),
`normalize_title`, the wire field, the FFI record and `NoteEdit::Title` /
`ClearTitle`, the web editor's "Title (optional)" input and the card's <h3>, and
the Android title field in both the compose sheet and the editor.

**The search vector had to be rebuilt, not just left alone.** `notes.search_vector`
is a STORED GENERATED column whose expression names `title` — Postgres refuses to
drop a column another generated column depends on. It is dropped and recreated over
`display_title` at weight A, which keeps the original intent: a note's NAME ranks
above the rest of its body.

**An imported title becomes the note's first body line.** Keep notes carry one, and
so does any ThoughtSync export taken before this. Dropping it would silently lose
text someone wrote; folding it in puts it exactly where a name now lives, so the
note arrives named as it was. Skipped when the body already opens with that line,
so re-importing an export this code produced doesn't stack duplicates.

Two smaller things fell out. The Android editor loses its bold first field — one
weight throughout, because the first line is the note's name but not a different
KIND of text, which is most of step 4 arriving early. And `ClearTitle`'s
justification comment moved to `ClearRemindAt`, which is now the surviving example
of why NoteEdit is a list rather than a struct of options.

Protocol note corrected to say what actually shipped: v2 is "no kind, no title",
one bump for the pair.

Verified with the local Rust gate this time, not by CI: fmt, clippy and 116 tests
all green before pushing. It caught four things — orphaned serde attributes where
fields were removed, a `wire::Preview.title` I deleted by mistake (a link preview
still has one), nine retention fixtures inserting a dropped column, and four
rustfmt diffs.

tests/test_notes.py

@@ -51,9 +51,10 @@ def test_all_note_routes_registered(app):
 
 def test_is_empty_note():
     assert is_empty_note(None, None)
-    assert is_empty_note("", "   ")
-    assert not is_empty_note("title", "")
-    assert not is_empty_note("", "body")
+    assert is_empty_note("   ", [])
+    assert not is_empty_note("body")
+    # A note that is only a checklist is not empty — it just has nothing in its body.
+    assert not is_empty_note("", ["milk"])
 
 
 def test_normalize_color():
@@ -69,9 +70,9 @@ def test_palette_has_core_colors():
 
 
 def test_serialize_shape():
-    n = Note(title="t", body="b", color="blue", pinned=True, archived=False)
+    n = Note(body="b", color="blue", pinned=True, archived=False)
     s = n.serialize()
-    assert s["title"] == "t"
+    assert "title" not in s  # there is no title field any more (M13 step 3)
     assert s["body"] == "b"
     assert s["color"] == "blue"
     assert s["pinned"] is True
@@ -122,30 +123,34 @@ async def test_reorder_requires_auth(app):
 # notice a route coming back, and the removal is one commit rather than a fossil.
 
 
-def test_derive_display_title_explicit_wins():
-    assert derive_display_title("My Title", "some body line") == "My Title"
-    assert derive_display_title("  Padded  ", "body") == "Padded"
-
-
-def test_derive_display_title_from_first_body_line():
-    assert derive_display_title(None, "first line\nsecond line") == "first line"
-    assert derive_display_title("", "  spaced first  \nnext") == "spaced first"
+def test_derive_display_title_is_the_first_body_line():
+    assert derive_display_title("first line\nsecond line") == "first line"
+    assert derive_display_title("  spaced first  \nnext") == "spaced first"
     # leading blank/whitespace lines are skipped to the first line with content
-    assert derive_display_title(None, "\n   \nreal line\nmore") == "real line"
-    # a whitespace-only title falls through to the body
-    assert derive_display_title("   ", "body wins") == "body wins"
+    assert derive_display_title("\n   \nreal line\nmore") == "real line"
+
+
+def test_derive_display_title_falls_back_to_the_first_item():
+    # What step 2 bought: a note that is only a checklist still has a name. Without
+    # this it would have none at all, which is why the title could not go first.
+    assert derive_display_title("", "milk") == "milk"
+    assert derive_display_title("   \n  ", "  eggs  ") == "eggs"
+    # The body still wins when it has anything to say.
+    assert derive_display_title("shopping", "milk") == "shopping"
 
 
 def test_derive_display_title_empty():
-    assert derive_display_title(None, None) == ""
-    assert derive_display_title("", "") == ""
-    assert derive_display_title("   ", "   \n  ") == ""
+    assert derive_display_title(None) == ""
+    assert derive_display_title("") == ""
+    assert derive_display_title("   \n  ", None) == ""
+    assert derive_display_title("   \n  ", "   ") == ""
 
 
 def test_derive_display_title_caps_length():
     long = "x" * 300
-    assert derive_display_title(None, long) == "x" * 200
-    assert derive_display_title(long, "body") == "x" * 200
+    assert derive_display_title(long) == "x" * 200
+    # the item fallback is capped on the same rule
+    assert derive_display_title("", long) == "x" * 200
 
 
 def test_parse_tags():
@@ -371,6 +376,8 @@ def test_native_spec_roundtrip_fields():
         "attachments": [{"file": "attachments/ab/img.png", "mime": "image/png"}],
     }
     spec = _native_spec(n)
+    # The spec still CARRIES a title — an export taken before M13 has one, and
+    # _create_imported_note folds it into the body rather than dropping it.
     assert spec["title"] == "T"
     assert spec["body"] == "b"
     assert spec["color"] == "blue"