Security: curl-pipe-bash installation pattern enables RCE if CDN is compromised

## Summary

Both the provider installation and fleet update mechanisms use the `curl | bash` pattern, which provides no integrity verification before executing downloaded code. A compromised or DNS-hijacked `api.darkbloom.dev` results in arbitrary code execution on every machine that installs or updates.

## Evidence

**README / provider install:**
```bash
curl -fsSL https://api.darkbloom.dev/install.sh | bash
```

**Fleet update script** (`deploy/provider-fleet/update-fleet.sh`):
```bash
ssh "$HOST" "curl -fsSL $COORD_URL/install.sh | bash"
```

The fleet script SSHs into each provider machine and pipes a remotely-fetched script directly to bash. A single compromised coordinator endpoint results in simultaneous arbitrary code execution across the entire provider fleet.

## Impact

**For individual providers:**
- A DNS hijack or CDN compromise of `api.darkbloom.dev` serves a malicious `install.sh`
- Executed immediately with the user's privileges
- No opportunity to inspect the script before execution

**For the fleet update path (higher severity):**
- Eigen Labs' own fleet update tooling uses `curl | bash` over SSH
- A coordinator compromise gives an attacker a one-command mechanism to execute arbitrary code on every enrolled provider machine simultaneously
- This is a supply chain attack that scales across the entire network

## Suggested Fix

**Option A: Signed install script (recommended)**

Sign `install.sh` with a GPG key or `cosign`, and verify before execution:

```bash
# Download script and signature separately
curl -fsSL https://api.darkbloom.dev/install.sh -o install.sh
curl -fsSL https://api.darkbloom.dev/install.sh.sig -o install.sh.sig

# Verify signature against pinned public key before executing
gpg --verify install.sh.sig install.sh && bash install.sh
```

Publish the public key fingerprint in this repository so users can pin it independently of the CDN.

**Option B: Homebrew / package manager distribution**

Distributing via `brew install darkbloom` or a signed `.pkg` installer provides OS-level integrity guarantees and eliminates the `curl | bash` pattern entirely.

**Option C: Hash verification with out-of-band hash**

At minimum, publish the SHA-256 of each `install.sh` release to GitHub Releases (separate trust root from the CDN) and instruct users to verify before running:

```bash
curl -fsSL https://api.darkbloom.dev/install.sh -o install.sh
echo " install.sh" | shasum -a 256 -c && bash install.sh
```

**For the fleet update script specifically:**

Replace the `curl | bash` SSH pattern with a versioned, pre-verified binary deployment:

```bash
# Instead of: ssh "$HOST" "curl -fsSL $COORD_URL/install.sh | bash"
# Use: scp verified_binary "$HOST:/usr/local/bin/darkbloom" && ssh "$HOST" "darkbloom update"
```

## References

- `deploy/provider-fleet/update-fleet.sh` -- fleet update curl-pipe-bash
- README installation instructions
- Related issue: #709 (circular binary trust -- the install script has the same CDN single-trust-root problem)
- Prior art: Homebrew's approach to signed formulae, Sigstore cosign for install scripts

Start with deploy/provider-fleet/update-fleet.sh and the installation command in README, then compare both paths with the related trust issue #709. Review the proposed verification and deployment approaches with maintainers; done means neither path executes code fetched from the CDN without an independent integrity check.

Written by the indexing model from the issue text.