PR #652 migrated sub-agent definitions from JSON+MD sidecar pairs to single
markdown files with YAML frontmatter, added an optional Context parameter to
spawn_agent, and enriched the [available-subagents] discovery context layer
to include per-agent description, tools, and an example call. None of that
was reflected in the runbook — anyone following the existing "Defining
subagents" section would write a .json file the loader rejects.

Rewrite docs/runbooks/subagents.md end to end:

- Discovery section shows the new enriched block (per-agent description +
  tools + timeout + a how-to-delegate block with the context field example)
  instead of the old one-line form.
- Invocation section documents all three spawn_agent arguments (agent, task,
  context) with one null-context and one context-populated example. Clarifies
  that Context is prefixed onto the subagent's first user message as a
  Context:/Task: block, and that the agent's system prompt stays verbatim
  from disk (not mutated by runtime context).
- Defining subagents section replaces the JSON schema + companion-file
  walkthrough with a single markdown-with-frontmatter template. Documents
  every frontmatter field with required/default columns: name, description,
  tools, modelRole, timeoutSeconds, visibility (hyphenated or PascalCase),
  emitStructuredFindings. Adds a loader-behavior-fail-loud subsection listing
  every rejection reason the scanner can log.
- Creating a custom agent walkthrough now writes a single .md file with
  frontmatter instead of a JSON+MD pair.
- Limitations section gains an entry noting that per-agent model selection
  is not yet supported and is blocked on the multi-model provider follow-on.

Also update IMPLEMENTATION_PLAN.md line 969 — the "Subagent Roadmap" entry
still listed the JSON file format. Updated minimally to reference the new
format and cite both #226 (original disk-based design) and #652 (migration
to frontmatter).

Historical references in RELEASE_NOTES.md are left alone — they accurately
describe what shipped at #226 and shouldn't be retconned.

PR #652 migrated sub-agent definitions from JSON+MD sidecar pairs to single
markdown files with YAML frontmatter, added an optional Context parameter to
spawn_agent, and enriched the [available-subagents] discovery context layer
to include per-agent description, tools, and an example call. None of that
was reflected in the runbook — anyone following the existing "Defining
subagents" section would write a .json file the loader rejects.

Rewrite docs/runbooks/subagents.md end to end:

- Discovery section shows the new enriched block (per-agent description +
  tools + timeout + a how-to-delegate block with the context field example)
  instead of the old one-line form.
- Invocation section documents all three spawn_agent arguments (agent, task,
  context) with one null-context and one context-populated example. Clarifies
  that Context is prefixed onto the subagent's first user message as a
  Context:/Task: block, and that the agent's system prompt stays verbatim
  from disk (not mutated by runtime context).
- Defining subagents section replaces the JSON schema + companion-file
  walkthrough with a single markdown-with-frontmatter template. Documents
  every frontmatter field with required/default columns: name, description,
  tools, modelRole, timeoutSeconds, visibility (hyphenated or PascalCase),
  emitStructuredFindings. Adds a loader-behavior-fail-loud subsection listing
  every rejection reason the scanner can log.
- Creating a custom agent walkthrough now writes a single .md file with
  frontmatter instead of a JSON+MD pair.
- Limitations section gains an entry noting that per-agent model selection
  is not yet supported and is blocked on the multi-model provider follow-on.

Also update IMPLEMENTATION_PLAN.md line 969 — the "Subagent Roadmap" entry
still listed the JSON file format. Updated minimally to reference the new
format and cite both #226 (original disk-based design) and #652 (migration
to frontmatter).

Historical references in RELEASE_NOTES.md are left alone — they accurately
describe what shipped at #226 and shouldn't be retconned.