skillscript-runtime v0.19.13

Removed (pre-v1.0 surface freeze cleanup)

Two frontmatter headers that shipped as discipline-only-contract
surfaces with no enforcement or downstream consumers — removed
to keep the v1.0 frozen surface honest.

• # Connectors: — documented in language-ref atoms but never
  parsed, linted, or read anywhere in the runtime. Doc surface
  claimed per-skill connector selection; implementation had zero
  lines of code matching the claim. Removed from doc atoms; no
  code change needed since nothing referenced the header.
• # Use when: — inverse rot: parsed and stored on the AST
  (ParsedSkill.useWhen) but with zero downstream readers (no
  skill_list surface, no lint, no dashboard render). Parser hook
  and AST field removed; existing skills using the header (if any)
  now silently ignore the line, same as any unrecognized
  # Foo: key.

Both were the same shape — surfaces named without wiring — and
the v1.0 surface-freeze commit means every documented header must
be backed by actual behavior. Pre-v1.0 + zero adopter usage =
clean removal, no migration tooling needed.

Documented — agent_id 2-level resolution chain

# Output: agent: <target> and # Output: template: <target>
have always resolved through a 2-level chain (explicit literal

• compile-time ${VAR} substitution against the resolved inputs
  map), but the spec lived only in language-ref atoms — no MD
  documentation, no help() topic coverage. Skill authors writing
  # Output: agent: ${TEAM} had to discover the behavior by
  trial-and-error or by reading compile.ts.

Now documented in:

• docs/language-reference.md — new "Output target resolution"
  subsection in the output section (right after Multiple output
  targets).
• src/help-content.ts — new "Output target resolution"
  subsection in the help(topic: "frontmatter") reference.

No code change; pure documentation completion of the existing
v0.19.12 surface ahead of the v1.0 freeze.