docs: split hook environment variables section into dedicated reference doc

[randlee]

Background

docs/agent-tool-use-best-practices.md grew significantly in PR #62 (+365 lines net). The bulk of the growth is in the environment variables / availability matrix section, which is dense reference material distinct from the best-practices guidance.

Recommendation

Extract the following into a dedicated docs/hook-environment-variables.md reference doc:

• Environment Variables: Availability by Context (availability matrix table)
• CLAUDE_PROJECT_DIR / CLAUDE_PLUGIN_ROOT / CODEX_PROJECT_DIR reference
• get_project_dir() / get_plugin_root() helper function patterns
• cwd vs stable root identity semantics

agent-tool-use-best-practices.md would keep a short summary and link to the dedicated doc for the full reference.

Benefits

• Best-practices doc stays focused on patterns and guidance
• Reference material is easier to find and cite independently
• Easier to update as new captures back or refine env var claims

Notes

• Evidence sourcing (schook captures, sc-test-harness baseline) should stay with the reference doc
• The availability matrix rows for Frontmatter Hook and Named Teammate are still marked untested — a good prompt to revisit once more captures land

🤖 Generated with Claude Code

[randlee]

Update: A Rust hook system is currently being built for future integration into synaptic-canvas plugins. This will generate capture-backed evidence for surfaces currently marked as untested in the availability matrix (CLAUDE_PLUGIN_ROOT, Named Teammate env vars, Frontmatter Hook contexts). The doc split in this issue should be scoped with that in mind — the reference doc will have more complete rows once the Rust hook integration lands.