fix: bump codebase-memory-mcp to v0.8.1 (#1284)

## Description

Bump `CBM_VERSION` from `v0.6.0` to `v0.8.1` in
`headroom/graph/installer.py`.

The v0.6.0 release assets are absent from GitHub — downloading the
darwin-arm64 binary (and likely other platform binaries) returns HTTP
404, making `--code-graph` unusable. v0.8.1 is the latest release with
all platform binaries present.

Closes #1283

## Type of Change

- [x] Bug fix (non-breaking change that fixes an issue)

## Changes Made

- `headroom/graph/installer.py`: `CBM_VERSION = "v0.6.0"` → `"v0.8.1"`

## Testing

- [ ] Unit tests pass (`pytest`)
- [ ] Linting passes (`ruff check .`)
- [ ] Type checking passes (`mypy headroom`)
- [ ] New tests added for new functionality
- [x] Manual testing performed

### Test Output

```text
Verified https://github.com/DeusData/codebase-memory-mcp/releases/tag/v0.8.1 contains
darwin-arm64, linux-arm64, linux-amd64, and windows-amd64 assets.
v0.6.0 tag/assets do not exist on that repo.
```

## Real Behavior Proof

- Environment: macOS darwin-arm64
- Exact command / steps: `headroom wrap claude --code-graph`
- Observed result: HTTP 404 on
`https://github.com/DeusData/codebase-memory-mcp/releases/download/v0.6.0/codebase-memory-mcp-darwin-arm64.tar.gz`;
v0.8.1 assets confirmed present at the new URL
- Not tested: actual end-to-end `--code-graph` run after bump (no local
headroom dev env)

## Review Readiness

- [x] I have performed a self-review
- [x] This PR is ready for human review

## Checklist

- [x] My code follows the project's style guidelines
- [x] I have performed a self-review of my code
- [ ] I have commented my code, particularly in hard-to-understand areas
— N/A, one-line version bump
- [ ] I have made corresponding changes to the documentation — N/A
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective — existing
test_graph.py covers download failure path; no new test needed for a
version constant bump
- [x] New and existing unit tests pass locally with my changes
- [ ] I have updated the CHANGELOG.md if applicable — left to
maintainers

## Additional Notes

The `CBM_VERSION` constant is the single source of truth for the
download URL. No other changes required.

headroom/graph/installer.py

@@ -13,7 +13,7 @@ from urllib.request import urlopen
 
 logger = logging.getLogger(__name__)
 
-CBM_VERSION = "v0.6.0"
+CBM_VERSION = "v0.8.1"
 CBM_REPO = "DeusData/codebase-memory-mcp"
 CBM_BIN_DIR = Path.home() / ".local" / "bin"
 CBM_BIN_NAME = "codebase-memory-mcp"