Replace loadout skill infrastructure with sk + groundwork

[pentaxis93]

Replace loadout skill infrastructure with sk + groundwork

Context

agentd currently vendors skills locally via a custom supply chain:
loadout config, manifest, sync script, verify script. The skills
themselves (ground, bdd, planning, land) are general-purpose
development methodology — not agentd-specific. They now live in
their canonical home: pentaxis93/groundwork.

Replace the custom infrastructure with sk (skills-supply), which
handles git-based skill distribution, multi-agent installation, and
reconciliation.

Prerequisites

• pentaxis93/groundwork repo exists with skills in place

What to remove

• skills/ directory (vendored skill copies)
• skills.manifest.toml
• scripts/skills_sync.sh
• scripts/skills_verify.sh
• .loadout/ directory
• Makefile targets: skills-verify, skills-sync
• Any README/docs references to the loadout workflow

What to add

• agents.toml at repo root:

[agents]
claude-code = true
codex = true
opencode = true

[dependencies.groundwork]
gh = "pentaxis93/groundwork"

• Document sk usage in README or CONTRIBUTING (one line:
  sk sync to install project skills)
• Update AGENTS.md skill routing to reflect sk-installed paths

What to update

• AGENTS.md: skill references must point to wherever sk installs
  them (agent-specific discovery paths, not skills/)
• Any CI that runs make skills-verify should be removed or
  replaced with sk sync --dry-run if drift detection is wanted

Acceptance criteria

• sk sync from a fresh clone installs all four groundwork skills
  into agent discovery paths
• No loadout artifacts remain in the repo
• AGENTS.md correctly routes agents to installed skills
• A contributor who has never seen this repo can run sk sync
  and start working