m4.5: inline #tags — hashtags in a note become labels (two-way sync)

Fast, cross-device labelling: type #groceries in a note and it becomes
the "groceries" label. The body is the source of truth for tag-labels;
manual picker labels stay independent (rule 28 — additive).

- note_labels.via_tag (migration 0013) marks tag-sourced attachments.
- parse_tags(): #tag at start-of-body or after whitespace, needs a
  letter (so #2024, URL #frags, mid#word are ignored). unit-tested.
- _reconcile_tags() on create + body-update: attach labels for current
  #tags (find-or-create, case-insensitive), detach tag-labels whose tag
  was removed; never touches manual (via_tag=false) rows.
- label picker (set_note_labels + editor onLabelsChange) now preserves
  tag-labels on save, so a picker action can't strip a label the #tag
  still mandates.
- serialize via_tag; card/editor chips render tag-labels as "#name",
  and the editor hides the × on them (remove by editing the tag text).
- LabelPicker builds manual NoteLabels (via_tag:false).

Fourth and final item of M4.5.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

tests/test_notes.py

@@ -9,6 +9,7 @@ from thoughtsync.notes import (
     normalize_color,
     parse_link_titles,
     parse_list_items,
+    parse_tags,
     rewrite_link_title,
 )
 
@@ -131,6 +132,16 @@ def test_derive_display_title_caps_length():
     assert derive_display_title(long, "body") == "x" * 200
 
 
+def test_parse_tags():
+    assert parse_tags("buy milk #groceries and #to-do now") == ["groceries", "to-do"]
+    # case-insensitive dedup, first spelling wins
+    assert parse_tags("#Work then #work") == ["Work"]
+    # url fragments, mid-word #, purely-numeric, and a bare # are not tags
+    assert parse_tags("frag http://x/#nope mid#word #2024 #") == []
+    assert parse_tags(None) == []
+    assert parse_tags("#a #b #a") == ["a", "b"]
+
+
 def test_parse_list_items():
     assert parse_list_items(["milk", " eggs ", "", "   ", "bread"]) == ["milk", "eggs", "bread"]
     assert parse_list_items("not a list") == []