docs: move audit-channel how-to from recipes to a maintainer ops runbook - #105

The out-of-band audit channel is maintainer-only (it runs through the
`ctxctl` binary, which ships to no end user), so its how-to does not
belong among the user-facing recipes. Relocate it to the operations /
contributor surface and make contributing.md the clear entry point.

- Move docs/recipes/audit-channel.md ->
  docs/operations/runbooks/audit-channel.md (git rename), beside the
  other maintainer runbooks (codebase-audit, docs-semantic-audit); fix
  its now-deeper relative links (banner image, context-health).
- contributing.md: the ctxctl section now states it covers only
  build/install and points to the runbook for the full workflow (a
  clear directive, not a skippable aside).
- recipes/index.md: replace the full entry with a short pointer to the
  runbook. Keeps an inbound redirect, removes it from user recipes,
  and clears the name overlap with recipes/system-hooks-audit.md.
- zensical.toml + operations/index.md: register the runbook under
  Operations -> Runbooks.
- specs/ctxctl-bootstrap.md: update the doc's path reference.
- Regenerate the docs site (site/**, feed); no orphaned recipe page.

Verification: `make lint` 0 issues (both modules), `make test` green
(both modules); all changed doc links resolve.

Spec: specs/ctxctl-bootstrap.md
Signed-off-by: Jose Alekhinne <jose@ctx.ist>