feat(agent-isolation): expose harness-agnostic agent-iso entry point

[justinmclean]

Summary

The env-isolation core of agent-iso.sh already worked for any CLI; this makes that capability explicit and documented:

• Add agent-iso() shell function (sourced) and a sub-command mode bash agent-iso.sh agent-iso <cli> [args] (direct exec) so any harness CLI can be launched with the same credential-strip posture that claude-iso and opencode-iso provide. A symlink named agent-iso (no .sh extension) also works — first positional arg is the CLI name.
• The Claude-specific --settings sandbox allowRead injection continues to be skipped for all non-claude agents (guard was already in place).
• Change **Harness:** Claude Code, OpenCode → **Harness:** agnostic in tools/agent-isolation/README.md; the env-isolation layer is now available for Codex, Cursor, Gemini CLI, Aider, and any future CLI.
• Update docs/vendor-neutrality.md harness table and harness index to reflect agent-isolation moving from portable to agnostic; the hook regenerates the score section automatically.
• Update docs/adapters/add-a-harness.md Step 2 and Step 4 to document agent-iso <cli> as the ready-made layer-0 wrapper for runtimes without a built-in hook API.
• Add tests/test_generic_iso.py covering both the direct-exec and sourced entry points: credential strip, no --settings injection, banner, missing-CLI exit-127, no-CLI-arg usage message, and multiple harness names (codex/cursor/gemini).

All 72 agent-isolation tests pass; vendor-neutrality-score and skill-and-tool-validate both pass.

Generated-by: Claude (claude-sonnet-4-6)

Type of change

• Skill change (.claude/skills/<name>/) — eval fixtures updated below
• Tool / bridge contract (tools/<system>/*.md)
• Python package (tools/*/ with pyproject.toml)
• Groovy reference impl
• Cross-cutting (RFC, AGENTS.md, sandbox, privacy-LLM)
• Documentation (docs/, README.md, CONTRIBUTING.md)
• Project template (projects/_template/)
• CI / dev loop (prek, workflows, validators)
• Other:

Test plan

• prek run --all-files passes
• For Python packages touched: uv run pytest / ruff check / mypy passes
• For Groovy bridges touched: command-line invocation tested end-to-end
• For skill changes: eval suite passes for the affected skill
  (PYTHONPATH=tools/skill-evals/src python3 -m skill_evals.runner tools/skill-evals/evals/<skill>/)
• For skill behaviour changes: a new or updated eval fixture is included in this PR
  (a regression test for the bug fixed / the behaviour added — see CONTRIBUTING.md)
• Other: