Add cross-process file locking for persistent OCI cache

[em-redhat]

Problem / Motivation

The persistent OCI cache introduced in PR #127 does not include cross-process file locking. While oci.Store from oras-go provides file-level atomicity for individual blob writes (content-addressable, write-to-temp-then-rename), concurrent CLI invocations or parallel agentic workflows operating on the same cache directory could encounter race conditions on:

• Simultaneous pull operations writing to the same cache
• A cache clean running while a pull or mcp serve is reading
• Multiple MCP server instances sharing the same cache directory

Proposed Solution

Add a cross-platform advisory lock (e.g., flock on Linux/macOS, LockFileEx on Windows) around critical cache operations:

1. Acquire a shared (read) lock before reading from the cache (loadBundleArtifacts)
2. Acquire an exclusive (write) lock before writing to the cache (oras.Copy, cache clean)
3. Use a well-known lockfile path within the cache directory (e.g., $CACHE_DIR/.lock)

Acceptance Criteria

• Concurrent complypack pull invocations do not corrupt the cache
• complypack cache clean waits for in-flight reads/writes to complete
• Locking works on Linux and macOS (Windows support can be a follow-up)
• Lock acquisition has a configurable timeout with a sensible default

Context

Raised by @trevor-vaughan in PR #127: #127 (comment)