Rename the essay skills to essay-outline/essay-draft and document the naming convention

## Summary

Rename the two essay-authoring skills to a `-` family and document the convention, so any later pipeline sits beside them coherently.

## Motivation

The naming complaint is unchanged and never depended on the taxonomy: `outline-draft` and `post-draft` are one pipeline but share no noun, and "draft" is a *stage* in the first name and a *verb* in the second.

What changed is that the family now has a natural head. `shape` (#598) gives each post a name — `essay`, `practice`, `review`, `how-to`, `note` — so the skill family is `-`, with `outline` as the approval gate and `draft` as prose. `review-draft` already fits it, which is a good sign the convention is the right one.

## Scope

- Rename `.claude/skills/outline-draft/` → `essay-outline`, `.claude/skills/post-draft/` → `essay-draft` (directory plus `name:`/`description:` frontmatter).
- Update cross-references: `essay-draft` → `essay-outline`, the `syndicate-instagram` reference to `post-draft`, the two `outlines/*.md` provenance mentions, the Skills section in CLAUDE.md, and the Drafting column in `docs/reference/post-shapes.md`.
- Document the `-` convention alongside the skill list.

## Acceptance criteria

- [ ] `/essay-outline` and `/essay-draft` invoke the renamed skills.
- [ ] No stale references to `outline-draft` / `post-draft` remain (`grep -rI` clean).
- [ ] The `-` convention is documented.

## Additional context

- Was blocked by #320, closed as not planned. The vocabulary this renames against is `shape`, settled in #598.
- `practice`, `how-to`, and `note` have no pipeline; `note-draft` is #323.