Add a Gemfile / Gemfile.lock parser (Ruby)

[AshwinUgale]

Why

Third format named in the README roadmap. Gemfile.lock has an exact-version GEM/specs section that maps cleanly onto Pin.

What to do

• Add parse_gemfile(path) to sidecar/src/docchat_sidecar/lockfiles.py.
• Prefer Gemfile.lock's specs: block (exact versions) when present; fall back to parsing gem "rails", "~> 7.0" lines in the Gemfile and running the range through _strip_range.

Acceptance criteria

• Tests for the lock-present and lock-absent paths plus malformed input.
• Full sidecar check pipeline green.

Pointers

• lockfiles.py: Pin, _strip_range, and the existing parsers as templates.