feat(gigacode): add AgentOS-backed OpenCode bridge

[NathanFlurry]

• Add the Gigacode experiment: an OpenCode-compatible local daemon and CLI backed by RivetKit AgentOS actors.
• Reuse one workspace actor per cwd, persist sessions through a coordinator, expose model/session/command compatibility, and add human-readable startup and per-session diagnostics.
• Harden ACP session lifecycle, configuration, ordering, cancellation, resume, and Claude/Pi adapter behavior needed by the OpenCode TUI.
• Add SDK, llmock, multi-turn/resume, terminal-driven, credential, mount, and compatibility coverage.

Validation:

• pnpm --dir experiments/gigacode check-types
• cargo check -p agentos-actor-plugin -p agentos-sidecar -p agentos-sidecar-core -p agentos-client
• pnpm --dir registry/agent/claude test (12 passed)
• pnpm --dir registry/agent/pi test (8 passed)
• Gigacode E2E suite previously passed 19/19 on this snapshot.

[railway-app]

🚅 Deployed to the agentos-pr-1748 environment in agentos

Service	Status	Web	Updated (UTC)
agentos	😴 Sleeping (View Logs)	Web	Jul 14, 2026 at 1:38 am

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🚅 Environment agentos-pr-1748 in rivet-frontend has no services deployed.