fix(peon-ping): auto-patch host.docker.internal on native Linux Docker

[baxyz]

Summary

• host.docker.internal doesn't resolve out of the box on native Linux Docker (only Docker Desktop injects that DNS entry automatically), which broke the audio relay unless the user manually added runArgs: ["--add-host=host.docker.internal:host-gateway"] to their own devcontainer.json — a Feature can't set runArgs itself.
• Adds a postStartCommand (patch-hosts.sh) that derives the same IP that runArgs flag would have resolved to — the container's own default gateway, read from /proc/net/route (no iproute2 dependency needed) — and writes it directly into the container's /etc/hosts, entirely inside the container's own filesystem namespace (confirmed: no host-side file is touched, no bind-mount of the host's /etc/hosts exists in this feature).
• Idempotent and best-effort: no-ops once it already resolves (including on Docker Desktop), falls back to sudo if a direct write is denied, only warns (never fails) if neither works — the manual runArgs fallback stays documented for that edge case.
• Verified locally: parsed gateway IP matches ip route's own output exactly (172.17.0.1 in this sandbox), the script no-ops correctly on a second run, and the sudo fallback path was exercised directly.
• Patch bump (1.2.2 → 1.2.3), not minor: no new capability for anyone who'd already added the runArgs workaround — this just removes the need for that step.
• Path double-checked against dotfiles-sync's identical, already-shipped pattern: install.sh copies to /usr/local/share/peon-ping/patch-hosts.sh, the manifest's postStartCommand points at that exact same path.

Board card: "Replace peon-ping's runArgs host.docker.internal fix with postCreateCommand pattern" (Backlog). The related card "Verify peon-ping host relay is actually reachable" needs real hardware to test and is out of scope here.

Test plan

• bash -n syntax checks, JSON validated
• Manual local verification of patch-hosts.sh (gateway parsing, idempotency, sudo fallback)
• Added test 8/9 to test/peon-ping/test.sh (script installed; host.docker.internal resolves, as a WARN not a hard FAIL since it's unconfirmed whether the test harness triggers postStartCommand before test.sh runs)
• CI green (39/40, then 40/40 after the version-fix push)

🤖 Generated with Claude Code

[github-actions]

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📋 Pipeline Status

	Job	Status
✅	🧾 Conventional Commits	passing
✅	🔖 Version Bump	passing
✅	🧪 Feature Tests	passing
✅	🐚 ShellCheck	passing

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_{🤖 Generated by @helpers4 CI • 2026-09-08}