feat(notation): Schlagworte &tag gebaut — taiga.<slug> benennt das Projekt je Teilbaum, vererbt (SPEC §1, D91)

SPEC zuerst: §1 Schritt 5b samt Block und Referenz-Regex, §11 auf
'umgesetzt' umgestellt; llms.md nachgezogen. Parser extrahiert
alleinstehende &-Token als node.marks (Zitier-Konvention gilt),
taigaSlugs() in model.js vererbt das taiga.*-Schlagwort wie die
@-Tags beim Engpass. Sichtbar im Tooltip und aria-label (a11yMarks,
9 Sprachen). Plan-Labels mit &-Erwähnungen eingeklammert; 12 neue
Tests, Gegenprobe: ohne Alleinstehend-Anker fallen genau die zwei
Zitier-Tests.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/CHANGELOG.md

@@ -19,6 +19,7 @@ reverse.
 
 ## 2026-08-27
 
+- Free `&tag` keywords are part of the notation now: free-standing only, quoted mentions like `(&taiga.slug)` stay labels — and `&taiga.slug` names the Taiga project of a subtree, inherited downwards
 - Plans spanning several Taiga projects: a `&taiga.slug` tag names the project per subtree, inherited downwards — the first consumer of the reserved free tags, recorded as planned
 - Creating Taiga tickets from nodes recorded as planned: a story per node from the node window, sub-packages as tasks picked in a dialog, through a backend proxy — the ref lands in the line as `#US-123`/`#T-1234`, beside the node id
 - The unassigned pill in the people bar shows each language's short form now — German `N.N.`, English `TBD` — with the long wording in the tooltip