feat: share Library configuration through Git

[tt-a1i]

Context

SkillRoster first releases are local-only. Each machine keeps its Library, Rosters, evidence, Plans, and Receipts locally.

A later version may let a user place a portable subset of Library configuration in Git so multiple machines can share declared Skill sources and desired Roster policy.

Proposed capability

• Export a portable, reviewable configuration without absolute local paths, session evidence, Plans, Receipts, or private prompts.
• Import or reconcile that configuration on another machine through a previewed Plan.
• Keep machine-specific paths and Agent availability local.
• Detect source revision conflicts and local overrides without silently overwriting either side.
• Document which fields are safe to commit and provide a conservative default .gitignore.

Safety requirements

• Opt-in only; no automatic push, pull, commit, or remote creation.
• Never include raw sessions, usage evidence tied to local history, secrets, or user-owned Skill contents unless explicitly selected.
• Import is read-only until the user approves the generated Plan.
• Existing local-only behavior remains the default.

Acceptance

Two clean machines can reconcile the same portable Library/Roster declaration while retaining different local roots, and the round trip exposes no machine-private data.

Not part of the first complete release

This issue is deliberately deferred until the local eight-Agent governance loop is complete and validated.