Add a go.mod lockfile parser (Go)

[AshwinUgale]

Why

Same motivation as the Cargo issue (#5) - Go is named in the README roadmap. go.mod is a simple line-oriented format, so this is a self-contained parser.

What to do

• Add parse_go_mod(path) to sidecar/src/docchat_sidecar/lockfiles.py. Parse require lines, both the single form require github.com/foo/bar v1.2.3 and the block form require ( ... ).
• The module path is the "library". Decide whether to keep or strip the leading v in the version (e.g. v1.2.3 -> 1.2.3) and document the choice in the docstring.
• Decide how to treat // indirect deps and document it. Return list[Pin].

Acceptance criteria

• Tests covering single require, block require, indirect deps, and missing/malformed files.
• Full sidecar check pipeline green.

Pointers

• Mirror parse_requirements_txt for the line-oriented parsing style.