rename: retire "pack" terminology in favor of "agent plugin" (APPENG-6571)

[r2dedios]

Summary

Retires the word "pack" repo-wide in favor of "agent plugin" / "plugin", per the OPL-approved naming in APPENG-6571 and the agent-plugins.org 1.0.0 spec. Scope: branded product names (agentic skill pack for X → agent plugin for X), prose/docs (CLAUDE.md, CONTRIBUTING.md, SKILL_DESIGN_PRINCIPLES.md, COLLECTION_SPEC.md, READMEs, Compass manifests, .catalog/), and functional code/CI identifiers (Makefile, scripts/*.py, GitHub Actions workflows).

• Pure rename — no logic/behavior changes, except one small safety addition: make validate-skill-design/validate-mcp-tools now fail fast with a clear message if invoked with the old PACK= variable instead of PLUGIN=, rather than silently validating everything.
• Fixes a pre-existing typo found along the way: rh-sre's catalog name said "Security Reliability Engineers" instead of "Site Reliability Engineers".
• deploy_and_use.md content (the APPENG-6004 install/usage rewrite) is untouched in substance — only the word "pack" was swapped to "plugin" within it.
• Independently re-verified with /code-review high: full validation suite passes (schema, compliance, structure, Compass manifests, lifecycle ceiling, docs links, Tier 1/2, MCP tools, both test suites, codespell); one mechanical find/replace artifact ("plugin plugin") was caught and fixed in a follow-up commit.
• Noted but intentionally not fixed here (pre-existing, unrelated to this rename): DEFAULT_PACKS/DEFAULT_PLUGINS in validate_docs_tree_links.py and validate_skill_doc_links.py is missing rh-basic — predates this PR, filing separately.

Plugin(s) affected

• Other / repo-wide

Change type

• Docs / README
• CI / tooling

Contribution method

• Manual contribution (validated with make validate + full validator suite)

Validation

• make validate passes locally (only unrelated pre-existing spell-check noise on untracked scratch files)
• pytest scripts/test_validate_lifecycle_ceiling.py scripts/test_validate_compass_layout.py — 27/27 pass
• All 7 packs' .catalog/collection.json verified byte-for-byte mirror of collection.yaml

🤖 Generated with Claude Code

[dmartinol]

/lgtm