docs: record skills.sh indexing status

[CodeSigils]

Project

• Repository or specification: CodeSigils/skill-discovery (documentation only — proposals/ROADMAP.md)
• Proposed category: Docs / implementation record

Why it belongs

Records the verified catalog-indexing status of this repository in the ROADMAP
implementation record, per the maintenance discipline rule that decisions are
recorded as one concise dated entry:

• Verified live on 2026-09-23 that the repository is already discoverable on
  skills.sh (detail page https://www.skills.sh/codesigils/skill-discovery
  returns 200 and lists the skill-discovery skill entry with install command
  npx skills add codesigils/skill-discovery).
• skills.sh is an open-source auto-indexer (vercel-labs/skills): repositories
  with the standard skills/ layout are crawled automatically; there is no
  manual submission flow.
• The README badge and "Catalog status" section are already accurate — no
  README change was needed.
• The Hermes hub (ZeroPointRepo/awesome-hermes-skills) was deliberately not
  pursued: it is a Hermes-Agent-specific curated list, and this agent-agnostic
  methodology skill is a poor fit; a contribution would require its own PR
  under that project's flow.

Quality evidence

• Verified live: skills.sh detail page returns HTTP 200; npx skills add codesigils/skill-discovery is the documented install path; the skill entry
  is listed.
• Docs-only change: 1 file, +13 lines (proposals/ROADMAP.md).
• No script, workflow, baseline, or CI-affecting changes.

Checklist

• One project per PR
• Read the contributing and roadmap guidance before editing
• Entry is a fact-based dated record, not marketing copy
• Formatting matches the file's existing implementation-record style
• Alphabetical ordering: N/A (dated entries, newest first)
• Description uses independently checkable facts
• Not a tutorial or commercial promotion
• No advisory-baseline / exception files touched
• All validation passes locally (docs-only change)
• I have not modified reviewer expectations for this PR