feat: bake pi agency toolchain (pi, pi-ledger, pi-tps) into runner image

[monotykamary]

Summary

Bakes the agency agent toolchain into the org self-hosted GitHub Actions runner image so headless dispatch workflows produce metered, notarized evidence without per-job installs.

• Dockerfile: installs @earendil-works/pi-coding-agent globally, plus the pi-ledger and pi-tps extensions into /etc/pi-agency/agent for any runtime user to seed. Uses the npm-published packages (the git-install path crashes npm's arborist on pi-ledger's pnpm workspace).
• agency-env.sh (/usr/local/bin): fails fast unless PI_LEDGER_IDENTITY_B64 (agency CI attestation identity) is present, seeds $HOME/.pi/agent from the baked copy, exports AGENCY_LEDGER_SESSION_DIR for evidence upload steps.
• README: documents the toolchain, the identity contract, and that autoscaler/ephemeral-replica behavior is unchanged.

Verification

• docker build green end-to-end; pi --version present in image; bash -n clean on both scripts.

Follow-ups

• Pin the pi version once the sealed-sidecar flow is smoke-tested on a real dispatch (pi-ledger declares a >=0.80.4 peer floor).
• Requires the PI_LEDGER_IDENTITY_B64 org secret scoped to repos running agent-dispatch.yml.