docs: fix the SKILLS.md heading hierarchy

[gregoryfoster]

Structural only — no fact in this doc changes.

The break

## The write-guard hook dangles on a submodule-less checkout sat between
### From gregoryfoster-skills and ### From obra-superpowers. An H2 in that position
terminates ## Available skills halfway through its own inventory, which makes the
obra-superpowers skill table a child of the write-guard section instead of the skills list.

Second, smaller: the v1.2 hold prose lived under ### From gregoryfoster-skills, a section whose
job is listing what that submodule provides. The hold is about submodule pinning and the suspended
refresh — adjacent, but not that.

After

# Agent Skills
## Vendor submodules
## Available skills
   ### Local overrides
   ### From gregoryfoster-skills
   ### From obra-superpowers
## Plans directory
## The curating-context v1.2 hold      <- lifted out of the table section
## Maintenance
## The write-guard hook dangles ...    <- moved down
## Submodules in CI

Reading order is now normal path first, exceptions after: maintenance, the hold that suspends part
of it, the hook that dangles without it, and CI.

The one substantive addition

The Maintenance block's git submodule update --remote --merge sat two lines above prose the
hold forbids, with nothing marking it. It now carries # SUSPENDED during the v1.2 hold — it would bump gregoryfoster-skills past the pin. Everything else is a cross-reference: the gregoryfoster
section points at the hold, and the CI section's "the v1.2 hold above" became a link now that it is
no longer directly above.

Verification

• Content-preservation check: sorted line-diff against HEAD shows only the intended additions —
  the new H2, two cross-reference lines, the SUSPENDED comment, and one bullet reflow. No prose lost.
• Anchors: all three intra-doc links resolve against the GitHub slug rules (#submodules-in-ci,
  #the-curating-context-v12-hold ×2). No inbound anchor links exist elsewhere in the repo — checked.
• codebase_context_index re-run; skills-doc re-chunked. Its manifest description still describes
  the file accurately, so it is unchanged.
• uv run ruff check . clean, ruff format --check clean, uv run pytest — 444 passed, 99.77%.

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