task: remove duplicated skill scripts copied across agents #3475

Part of #3464. Related: #3467, #3468.

Problem

Skill scripts are copied into many agents instead of kept once and referenced by path.

Some files exist 14 times (for example merge_docx_append.py, comments_strip.py, xlsx_to_docx_table.py, redact_docx.py, flatten_ref_fields.py), mostly under agents/*-agent/skills/**.

Impact: GitHub Code Quality re-analyses every copy (the Python scan takes about 125 s per PR, the slowest check); fixes must be applied up to 14 times, and copies drift. The external repo review flagged the same failure mode ("packaged copies of skills diverged; one copy, referenced by path").

Change

• Keep one canonical copy per skill under skills/<name>/ (Agent Skills layout).
• Agents reference skills by name or path instead of copying them; plugin marketplace entries reference canonical paths (aiops: distribute agents and skills org-wide via a plugin marketplace #3468).
• Add a CI check that fails on new duplicated skill files.

Acceptance

• Duplicate-copy count reported by the check drops to 0 (or an agreed allowlist)
• Code Quality Python analysis time measured before and after