Rewrite 548 skill descriptions to the activation rubric

Each rewritten description now states both what the skill does (concrete
capability, named tools/artifacts) and an explicit when-to-use trigger,
improving agent discovery/activation. Grounded in each skill's own body;
changes confined to the `description` field only (bodies and all other
frontmatter untouched). Produced by a gated audit->rewrite->recheck loop
(548 -> 0 flagged) with a sampled anti-invention check (0 ungrounded).

Schema: 817/817 pass. Framework-ID gate: 0 defects.

skills/performing-purple-team-atomic-testing/SKILL.md

@@ -1,12 +1,10 @@
 ---
 name: performing-purple-team-atomic-testing
-description: 'Executes Atomic Red Team tests mapped to MITRE ATT&CK techniques, performs
-  coverage gap analysis across the ATT&CK matrix, and runs detection validation loops
-  to measure blue team visibility. Covers Invoke-AtomicRedTeam PowerShell execution,
-  ATT&CK Navigator layer generation for heatmaps, Sigma rule correlation, and continuous
-  atomic testing pipelines. Activates for requests involving purple team exercises,
-  atomic test execution, ATT&CK coverage assessment, detection engineering validation,
-  or adversary emulation testing.
+description: 'Executes Atomic Red Team tests mapped to MITRE ATT&CK via Invoke-AtomicRedTeam
+  PowerShell, generates ATT&CK Navigator coverage heatmaps, correlates results against
+  Sigma rules, and runs detection validation loops to measure blue team visibility.
+  Use for purple team exercises, atomic test execution, ATT&CK coverage assessment,
+  or detection engineering validation.
 
   '
 domain: cybersecurity