refactor(mounts): extract host bind-mount logic into sandbox/mounts package

[neurolabs]

Description

Refactors the mount subsystem introduced in #42 out of internal/sandbox/options into its own internal/sandbox/mounts package, mirroring the existing internal/sandbox/network precedent for self-contained, domain-specific config types.

Triggered by discussion on #42, the hand-rolled mount shape parser (decodeMount switch) is replaced with mapstructure decoding: a string -> BindMount decode hook handles the short form (target: ~/.m2), and ResolveBindMounts remains the validation layer (path existence, ~ expansion, target conflicts, duplicates). Decoding is fully lenient — unknown fields are ignored.

The managed mount-point constants (VMHomeDir, WorkspaceMountPath, TmpMountPath) move with the subsystem so the dependency direction stays options -> mounts, and options no longer imports mapstructure.

Why not decode via viper's Unmarshal

A note for reviewers: the mounts map is decoded from v.Get("mounts") rather than through viper.Unmarshal because viper flattens dotted guest paths (e.g. /home/dev/.m2 into nested keys), which would corrupt the map keys. This is documented on the Config.Mounts field.

Checklist

• make check passes (fmt, lint, test)
• Tests added/updated
• Internal refactor only — no behavior change, so no README/docs/CHANGELOG update warranted

[codecov]

Codecov Report

❌ Patch coverage is 93.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.73%. Comparing base (b13e77b) to head (69c70f9).

Files with missing lines	Patch %	Lines
internal/sandbox/mounts/mounts.go	88.23%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #44      +/-   ##
==========================================
- Coverage   88.80%   88.73%   -0.07%     
==========================================
  Files          87       87              
  Lines        5314     5299      -15     
==========================================
- Hits         4719     4702      -17     
- Misses        413      414       +1     
- Partials      182      183       +1     

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