Add Devin plugin manifest and skills/unslop layout - #11

Adds a Devin plugin manifest so this repo installs directly as a Devin plugin — same layout Devin's own plugin packaging produces for unslop.

• .devin-plugin/plugin.json — name/version/description/author manifest.
• skills/unslop/ — symlinks (SKILL.md, references/, presets/, scripts/, evals/) into the skills/<name>/ layout Devin loads, with zero duplication of the canonical files. This mirrors the repo's existing symlink-based install approach (ln -s ~/dev/unslop ~/.claude/skills/unslop).
• README.md — one subsection documenting the Devin install.

No source or eval content changes — the symlinked entries are the same files. evals/check.py (fast lane), scanner contract, coverage, and taboo parity all pass unchanged.

Specimen & source

n/a — no scanner or catalog change.

Red-first proof

n/a.

Coverage

Gate results

$ python3 evals/check.py
  PASS 5  XFAIL 0 (known bugs)  XPASS 0 (fixed — remove xfail)  FAIL 0 (regressions)
PASS all checks

Verified on this branch; identical result on main (no eval/scanner
files touched). Pre-existing check.py --full ENC-01 dash-printf failure
noted for awareness — unrelated.

Notes for the reviewer

• skills/unslop/ entries are symlinks into the repo root, so the
  packaged layout and the repo share one copy of the content — the
  plugin can't drift from upstream. If a different layout is preferred
  (e.g. pointing skills at the root, or a vendored copy), happy to
  adjust.

Confirmation

• Specimen publication was approved by the source; redactions (if any)
  keep the tell byte-for-byte intact. — n/a, no specimen.
• The FN row was verified red before the fix. — n/a.
• No case was marked xfail to grandfather a gap; FP-06 remains the
  only documented XFAIL.