fix(ci): .clusterfuzzlite/Dockerfile symlink -> Containerfile (#87)

## Problem

The `google/clusterfuzzlite` `build_fuzzers` action
(`.github/workflows/cflite_pr.yml`) hardcodes the build path
`.clusterfuzzlite/Dockerfile` and exposes **no** filename input. The
repo ships the SPDX-headed `.clusterfuzzlite/Containerfile`, so the
**"PR (address)"** check fails:

```
ERROR: failed to solve: failed to read dockerfile: open Dockerfile: no such file or directory
```

Pre-existing (no `Dockerfile` on `main`); surfaced during #73 triage.

## Fix

Add a **tracked symlink** `.clusterfuzzlite/Dockerfile → Containerfile`
(git mode `120000`). clusterfuzzlite's
`open(".clusterfuzzlite/Dockerfile")` resolves through it, while the
canonical file keeps the house-style `Containerfile` name
(Podman-not-Docker estate rule, `.claude/CLAUDE.md`). Zero blast radius
— nothing references `.clusterfuzzlite/Containerfile` by name (Justfile
points only at `.containerization/Containerfile*`).

**Not a required status check** — does not block merge; this is
correctness/hygiene so the fuzz job actually runs.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

.clusterfuzzlite/Dockerfile

@@ -0,0 +1 @@
+Containerfile