feat: rename icontext → fbrain; deprecate icontext CLI/skills (v0.5.0)

[federicodeponte]

Summary

Internal rename to match the GitHub-level rename (icontext → fbrain) done on 2026-05-05. Adds backward-compat shims so existing users don't break.

What changed

• 51 files, 1142+/1056-
• Python package: name = "fbrain", version bumped to 0.5.0
• CLI: fbrain is now primary; icontext kept as deprecated shim that prints warning and forwards to fbrain
• Skills: 4 new fbrain-* skills added; old icontext-* SKILL.md files reduced to redirect stubs ("Renamed to fbrain-X. Will be removed in v0.6.0.")
• README, CHANGELOG, install.sh, get.sh, uninstall.sh, hooks, configs, MCP server, tests all updated

Self-audit (per Codex)

• ✅ python -m pytest -x → 105 passed
• ✅ Editable install verified in clean venv
• ✅ fbrain --version → 0.5.0
• ✅ icontext --help → prints deprecation warning then succeeds

What's preserved (sacred)

• ~/context/ data directory (user vaults untouched)
• demo/icontext-demo.gif and .cast filenames (assets, not user-facing)
• .icontext/manifest.json install manifest path (would break uninstall otherwise)
• ICONTEXT_VAULT env var still recognized

Deprecation timeline

• v0.5.0 (this PR): icontext CLI + icontext-* skills work but warn
• v0.6.0 (next minor): old aliases removed

Test plan

• On a clean machine: pip install -e . then fbrain init succeeds
• icontext init prints deprecation warning then succeeds
• Existing user installs still work (no manifest break)
• Slash commands /icontext-populate-profile and /fbrain-populate-profile both resolve