chore(skill): Own tttk-authoring in the repo

Move the skill to .agents/skills/tttk-authoring and point gen:skill-reference at it. Activate it by symlinking ~/.agents/skills/tttk-authoring to the in-repo copy.

Extend the reference drift guard to cover the skill copy now that it is versioned, and document activation in docs/development.md.

.agents/skills/tttk-authoring/examples/adventure.md

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+# The Hollow Mill
+
+## Adventure brief
+
+The mill on the village edge has stood silent for a year. Last week its wheel
+began turning again — on its own.
+
+> [!WARNING]
+> The mill's guardian is not hostile by default. Attacking it forfeits the
+> peaceful resolution.
+
+## Locations
+
+:md-table[./locations.csv]
+
+## Random encounters
+
+:md-table[./encounters.csv]{roll=true}
+
+## The mill guardian
+
+:md-dice[1d20+3]{key="guardian-attack"}
+
+The guardian strikes with :[sword] for :md-dice[2d6+2]{key="guardian-damage"}.
+
+## Encounter table (inline)
+
+```csv role=spark-table roll=true
+d6,Encounter
+1,A crow watches from the rafters.
+2,The floorboards groan underfoot.
+3,A cold draft carries the smell of grain.
+4,The wheel outside stops for one breath.
+5,A sack of flour splits open.
+6,The guardian's lantern flickers to life.
+```
+
+## Route
+
+```mermaid
+graph TD
+    A[Village] --> B[Mill Road]
+    B --> C{Mill Door}
+    C -->|Force| D[Ground Floor]
+    C -->|Listen| E[Secret Cellar]
+    D --> F[Wheel Room]
+    E --> F
+```
+
+## Guardian card
+
+```yaml role=tag
+tag: md-deck
+body: ./guardian.csv
+data-config:
+  size: 63x88
+  grid: 5x5
+  layers:
+    - prop: name
+      pos: 1,1-5,1
+      font: 12
+    - template: |
+        **{{type}}** — {{hp}} HP
+        {{description}}
+      pos: 1,3-5,8
+      font: 3
+      align: l
+```