feat: make skills platform-agnostic for cross-tool compatibility

[nathanschram]

Phase 2: Portable Skills (v1.7.0)

Context

ContextDocs skills reference Claude Code-specific concepts (/contextdocs: prefixes, @AGENTS.md imports, .claude/settings.json). Other tools (Codex CLI, Gemini CLI, Cursor, Copilot, Windsurf, Cline) now support the Agent Skills standard (SKILL.md) but can't use our skills because the instructions assume Claude Code.

Deliverables

1. Refactor ai-context SKILL.md — Remove /contextdocs: prefixes, use tool-neutral language ("run the ai-context command"), add Platform Notes section
2. Refactor context-verify SKILL.md — Platform-neutral framing for verification checks
3. Refactor context-guard SKILL.md — Split into hook-based workflow (Claude Code, Gemini, Copilot, Cursor) and manual workflow (Codex, OpenCode, Cline, Windsurf)

Design Decisions

• Use conditional sections within single SKILL.md rather than per-platform copies
• Keep core workflow (Signal Gate, AGENTS-first, line budgets) unchanged — already tool-agnostic
• Claude Code-specific features (hooks, Import (A place for python bower components) (@import)) noted as platform-specific caveats

Verification

• Skills still trigger correctly in Claude Code (interactive mode)
• Token budgets remain within limits
• validate-llms-txt.sh passes

Part of the Cross-Platform ContextDocs Strategy. Depends on #17.