docs(context): record that hand-invocation blocks skill-to-skill routing

The Hand-invoked skill entry covered listing visibility and preload tax but not
invocability. Measured: disable-model-invocation: true hard-blocks the Skill
tool, not just the model-visible listing, so a `Call `x`` step in another
skill's body stops working the moment x takes the flag.

ADR-0020's 'Invocation as a design axis' verified only the visibility and slash
directions. Without this half, marking grill-with-docs hand-invoked -- one of
issue #99's four named candidates -- would silently break forge Step 1, which
calls it, plus three other inbound routes.

Refs #99

CONTEXT.md

@@ -35,7 +35,9 @@ _Avoid_: router body, thin body
 **Hand-invoked skill**:
 A skill reached only by typing its slash command, declared `disable-model-invocation: true`. The host
 withholds it from the model-visible listing entirely, so it pays no preload tax and its description
-becomes human-facing text. Exemplar: `zoom-out`.
+becomes human-facing text. The flag also hard-blocks the Skill tool, so **no other skill can route to
+a hand-invoked skill** — a `Call \`x\`` step in another skill's body stops working the moment `x`
+takes the flag. Check inbound routes before declaring one. Exemplar: `zoom-out`.
 _Avoid_: manual skill, disabled skill
 
 **Delegation discipline**: