feat(core): add content-guide.md to agentsmd-author

PR review feedback: Step 3 gave no concrete guidance on what good
AGENTS.md content looks like, and the skill had no substantive
references file (only provenance bookkeeping in sources.md), unlike
sibling kyberforge skills. Adds section-by-section content guidance,
the worked example, and monorepo precedence rules synthesized from
the agentsmd research corpus.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

plugins/core/skills/agentsmd-author/SKILL.md

@@ -22,7 +22,7 @@ metadata:
     - agents-md-official
     - context7-websites-agents-md
     - context7-agentsmd-agents-md
-  version: "0.1.0"
+  version: "0.1.1"
 ---
 
 ## Gotchas
@@ -45,7 +45,7 @@ Before writing anything, gather real facts: package manager and scripts (`packag
 
 ## Step 3 — Write or update
 
-Use only sections that reflect something real about the repo. Common sections from the agents.md standard — setup/build commands, code style, testing instructions, security considerations, commit/PR conventions — are a checklist to consider, not a template to fill blindly. Prefer concrete commands and file paths over general advice.
+Use only sections that reflect something real about the repo — never fill in every common-sections-checklist heading just because it exists. Read `references/content-guide.md` for section-by-section guidance, a worked example, and what separates useful content from generic padding, before writing.
 
 ## Step 4 — Check for an existing provider file