feat(skills): gate installer behind install feature

[M09Ic]

Background / Motivation

We are targeting a minimal moltis agent build for edge-device environments. The HTTP/TLS stack pulled by reqwest has a significant binary-size impact, so installer-related networking should be feature-gated and only enabled when needed.

In our local minimal-path validation (cerebellum release), binary size dropped from 8,167,936 bytes to 5,128,192 bytes (-37.22%). This PR is the first foundational step: make the skills installer an explicit feature.

Changes

• crates/skills/Cargo.toml
  • Make reqwest optional.
  • Add install = ["dep:reqwest"].
  • Keep default = ["install"] to preserve existing behavior.
• crates/skills/src/lib.rs
  • Gate install module behind #[cfg(feature = "install")].

Compatibility

• Default behavior remains unchanged.
• Consumers can now disable install/http dependencies by turning off default features.

Validation

• cargo check -p moltis-skills
• cargo check -p moltis-skills --no-default-features
• cargo check -p moltis-skills --no-default-features --features install