refactor(core): retrofit agentsmd-audit to the ADR-0020 context contract

Description 944 -> 322 chars, Gotchas 36% -> 22%. The composition note
moves to README.md.

Restores the hand-edit trigger, which a clean-context audit found had no
other caller: agentsmd-author owns the post-authoring invocation, but a
hand-edit has no author skill in the loop, so nothing invoked the audit
at all. It survived only in README.md, which neither the router nor the
invoked agent loads. That is the path on which a human pastes a
credential into AGENTS.md.

The first pass dropped it against a measured budget of '~9 spare chars'.
The real cost was ~49, and 250 is the SUGGESTION tier, not a ceiling --
the gate fails at 400. Ships at 322 with one advisory line.

Names the three audit dimensions in the capability clause, recovering
routing for 'does my AGENTS.md leak credentials', and qualifies the
'is this AGENTS.md safe to commit' phrasing, whose pronoun had no
antecedent inside the quoted string.

Refs #99

plugins/core/.apm/skills/agentsmd-audit/README.md

@@ -14,6 +14,9 @@ Runs a single combined pass across every AGENTS.md file in a repo (root and any
 
 Provide the path to the repo root to audit when invoking.
 
+Also invoke it proactively after `agentsmd-author` creates or updates an AGENTS.md, or after a
+hand-edit made outside `agentsmd-author` — the audit is what confirms the result is safe to commit.
+
 ## Files
 
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