chore: acknowledge reviewed skill-budget warning

[CodeSigils]

Adopts the designed advisory-baseline mechanism for the new skill-budget
cron-health warning rather than trimming the shipped payload.

What changed

• advisory-baseline.json: reviewed 2026-09-15; records the 380-line
  SKILL.md warning as a known, accepted advisory.
• proposals/ROADMAP.md: new 2026-09-15 entry documenting the decision.

Rationale

skills/skill-discovery/SKILL.md is 380 lines against the 350-line warning
threshold. The content is dense guardrail and reporting-contract prose — not
recoverable bloat — and is stable since the last payload edit (2026-09-09).
The 350-line threshold remains active for future regression detection; the
exact-string baseline still flags any growth beyond 380 lines.

Validation

UV_CACHE_DIR=/tmp/skill-discovery-uv-cache uv run python .github/scripts/validate-docs.py
# PASS: documentation and shipped payload are valid

UV_CACHE_DIR=/tmp/skill-discovery-uv-cache uv run python scripts/cron-health.py
# ℹ️ 1 known warning(s) suppressed by baseline, exit 0

UV_CACHE_DIR=/tmp/skill-discovery-uv-cache uv run python scripts/check-readme-tree.py
# PASS: all 45 files in README tree exist on disk, no unlisted files

git diff --check
# clean