docs: add freview repository audit skill and accessibility guidance

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Purpose

Introduce freview — Full Repository Review, an on-demand, cross-agent read-only repository audit that checks real implementation/release status rather than trusting a Done label, plus public accessibility reporting guidance.

Added

• .agents/skills/freview/SKILL.md: portable skill compatible with agents that discover project skills; evidence-first issue → PR → code → test → exact-SHA CI → physical QA → release audit.
• .agents/skills/freview/references/review-matrix.md: branch-history, worktree, dependency graph, community standards and acceptance matrix.
• scripts/dev/freview.mjs: no-dependency, read-only local Git, manifest and hygiene snapshot, JSON/text output.
• .github/ACCESSIBILITY.md: honest accessibility goals, terminal-specific limitations and reporting path.
• docs/development/freview.md: how to invoke and interpret the audit; dependency graph versus network graph; comparable tools.

Safety/ownership

Documentation and standalone script only. No production NMSh code, versions, config, release or CI changed. No cleanup, branch deletion, merges or automatic GitHub issue edits. Based on dev at 4a02520d; independent of current release and post-v0.20 worktrees.

Verification

• Confirmed files committed on this draft branch.
• Separate local copy of the script passed Node syntax check and representative clean/dirty/multiple-worktree fixtures; exact pushed GitHub version needs its own syntax/runtime check before merge.
• No canonical NMSh build/tests or physical terminal QA were run (docs/read-only script; live coding agents active).

Review requested

• Check skill discovery path with actual Codex/OpenCode/Gemini/Claude installations.
• Verify node --check scripts/dev/freview.mjs and run read-only snapshot against the branch.
• Review accessibility language against current supported behaviors and any existing accessibility issue(s).
• Keep this PR draft until accepted. No release/tag or issue-state modifications.