docs: document all builtin hooks in schema and hooks page

[dgageot]

The hooks page and JSON schema only mentioned three builtins (add_date,
add_environment_info, add_prompt_files) even though the runtime now ships
nine. This brings the docs back in sync with pkg/hooks/builtins.

Changes:

• agent-schema.json – HookDefinition.type description now lists every
  shipped builtin with its target event and arg semantics; args description
  was extended with concrete examples for builtins that accept parameters
  (add_prompt_files, add_recent_commits, add_git_diff, max_iterations).
• docs/configuration/hooks/index.md – New Built-in Hooks section
  with a YAML example, a reference table of all 9 builtins (event + args +
  behavior), and callouts explaining per-turn vs. per-session semantics, the
  agent-flag shorthands (`add_date: true` etc.), and the difference between
  the `max_iterations` agent field and the same-name builtin hook.
• `examples/hooks.yaml` – Replaced the stale "Three are shipped"
  comment with the full list of shipped builtins and their events/args.

Validation: `go test ./pkg/config/... ./pkg/hooks/...` (incl.
`TestJsonSchemaWorksForExamples`).