security: verify asset checksums in install.sh before execution - #83

> **🏗️ build-from-issue-agent**

Closes NVIDIA#71

## Summary
Add dynamic shell completion support using `clap_complete`'s `CompleteEnv`, plus a `navigator completions <shell>` subcommand (rustup-style) that outputs completion scripts to stdout with detailed per-shell setup instructions in `--help`.

Completions are dynamic — the shell calls back into the `navigator` binary on each tab-press, so they are always in sync with the installed version. This also lays the groundwork for future runtime completers (e.g., listing sandbox names on tab, tracked in NVIDIA#83).

## Changes Made
- `Cargo.toml`: Added `clap_complete = { version = "4.5", features = ["unstable-dynamic"] }` to workspace dependencies
- `crates/navigator-cli/Cargo.toml`: Added `clap_complete` dependency
- `crates/navigator-cli/src/main.rs`:
  - Added `CompleteEnv::with_factory(Cli::command).complete()` at the top of `main()` (before TLS/argument parsing)
  - Added `Completions { shell }` subcommand that re-invokes the binary with `COMPLETE=<shell>` and pipes the output
  - Added `CompletionShell` enum (bash, fish, zsh, powershell)
  - Added `COMPLETIONS_HELP` with per-shell setup instructions (similar to `rustup completions --help`)
  - Added 3 unit tests
- `CONTRIBUTING.md`: Added shell completions section documenting `nav` wrapper setup

## Usage

```bash
# Generate completions
navigator completions fish > ~/.config/fish/completions/navigator.fish
navigator completions bash > ~/.local/share/bash-completion/completions/navigator
navigator completions zsh > ~/.zfunc/_navigator
navigator completions powershell >> $PROFILE

# See full per-shell instructions
navigator completions --help

# For the nav wrapper (rewrite registration target)
navigator completions fish | sed 's/--command navigator/--command nav/' > ~/.config/fish/completions/nav.fish
```

## Design Decision: Why stdout-only (no auto-install)

We researched how 12 popular CLI tools handle completion installation:

| Tool | Command | Writes files? | Modifies RC? |
|------|---------|---------------|-------------|
| gh, glab, kubectl, docker, rustup, mise, starship | `completion <shell>` | stdout only | No |
| atuin | `gen-completions --shell <shell>` | stdout (`--out-dir` option) | No |
| gcloud | `install.sh` | Yes | **Yes** |
| Amazon Q | `q integrations install` | Yes | **Yes** |

**10 of 12 tools** follow the same pattern: output to stdout, never modify RC files, explicit shell selection, clear `--help` instructions. Only gcloud and Amazon Q modify RC files (and both have been criticized for it). Our approach matches the industry consensus.

## Deviations from Plan
- The original issue proposed a static `completions <shell>` subcommand using `clap_complete::generate()`. After discussion, we switched to dynamic `CompleteEnv` which is always up-to-date and enables future runtime completers.
- Added a `navigator completions <shell>` subcommand (not in original plan) for discoverability — it wraps the `COMPLETE` env var mechanism with a familiar CLI UX.

## Tests Added
- **Unit:** `cli_debug_assert` — validates CLI structure; `completions_engine_returns_candidates` — smoke test confirming the completion engine returns subcommand candidates; `completions_subcommand_appears_in_candidates` — verifies `completions` appears in tab-completion results
- **Integration:** N/A
- **E2E:** N/A

## Verification
- [x] All tests passing
- [x] Pre-commit checks passing (clippy warnings are pre-existing, not from this change)
- [x] CONTRIBUTING.md updated with nav wrapper completion setup

* feat(skills): create nemoclaw-cli agent skill and fix generate-sandbox-policy schema drift

Closes NVIDIA#83

Create a new nemoclaw-cli agent skill with tiered workflow guidance (basic
through advanced) and a supplementary CLI reference. The skill covers provider
management, sandbox lifecycle, policy iteration loops, BYOC pipelines, and
agent-assisted sandbox sessions. It cross-references generate-sandbox-policy
for policy content authoring and includes --help self-teaching as a fallback.

Fix critical schema drift in generate-sandbox-policy: rename
allowed_routing_hints to allowed_routes (matching the actual proto/serde
field name). Update default read_only paths to match dev-sandbox-policy.yaml.
Fix the same field name error in architecture/security-policy.md.

* policy updates

* fix(test): update OPA tests for renamed github_ssh_over_https policy key

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Co-authored-by: John Myers <[email protected]>