feat: implement grip object model CLI (grip#606)

[laynepenney]

Summary

• Bridge grip_cli.py to match TDD spec contract in test_grip_object_model.py (14 tests, all previously failing)
• Auto-discover repos from workspace_spec.toml when --repos not provided
• Add JSON index layer at .grip/snapshots/index.json for O(1) lookup alongside git plumbing storage
• Dirty-repo blocking for snapshot and checkout commands
• Track detached HEAD state and empty repos in snapshot metadata

Details

The existing test_grip_snapshot.py (35 tests) tested the library API directly and all passed. The test_grip_object_model.py (14 tests) tested through the CLI via typer CliRunner and expected a different interface:

1. Repo auto-discovery from workspace_spec.toml (not --repos required)
2. JSON index storage at .grip/snapshots/index.json
3. Snapshot entries with id, message, repos, repo_states containing head, head_state, is_empty
4. Dirty-repo blocking for both snapshot and checkout
5. Clean error messages for missing .grip/ state

This implementation reconciles both contracts: keeps git plumbing internally (content-addressable), adds JSON index as a view layer, makes --repos optional with workspace_spec fallback.

Test plan

• All 14 test_grip_object_model.py tests pass (previously 0/14)
• All 23 test_grip_cli.py tests pass (no regressions)
• All 35 test_grip_snapshot.py tests pass (no regressions)
• Full gr2 test suite: 471 passed, 0 failed
• ruff lint clean
• cargo fmt clean

Premium boundary: core OSS (workspace orchestration, git plumbing).

Closes #606

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