docs(skills/ccc): document `ccc describe` and `ccc guide` (#165)

Adds two sections to the `ccc` skill:

- `Describing Files and Directories` — `ccc describe <path>` for
  per-file/dir summaries when the path is known.
- `Concept Guides` — `ccc guide [slug]` for cross-cutting
  architectural docs (`.cocoindex_code/guides.yml`), plus an
  "Authoring guides.yml interactively" subsection that gives the
  agent a 5-step collaborative flow (survey → propose → iterate →
  write → generate) and a YAML schema reference.

Inserts a `Following Hints in Search Output` subsection under
search: when a hit is tagged `[summary]` or `[guide]`, follow the
embedded `ccc describe`/`ccc guide` hint rather than running those
commands proactively as a triage step. This reflects empirical
finding that explicit guide-tier triage hurts (matched-vs-unmatched
guide reads are net-neutral in aggregate); search-driven discovery
is strictly better (~−34% tools, −26% time vs no-ccc on a 10-Q
battery).

Frontmatter trigger phrases extended to include `describe this
file` and `read the concept guide`.