feat(acpbridge): tailnet-only ACP-over-WebSocket bridge daemon

[enekos]

Summary

• New mairu acp-bridge daemon: tailnet-only WebSocket server that proxies ACP JSON-RPC frames between remote clients (mobile app, future tooling) and locally-spawned ACP agents (mairu acp, claude-code --acp, gemini --acp).
• Per-session event ring buffer + Last-Event-ID replay so reconnects don't lose frames.
• Server-initiated session/request_permission is fanned out to every attached client; first responder wins, 60s fallback synthesizes a denial so the agent never blocks indefinitely.
• Pluggable PeerAuthorizer interface with AllowAll (dev) and TailscaleAuth (production-bound) implementations. tsnet.Server embedding is deferred — see plan note 1.
• CLI: mairu acp-bridge --addr <host:port> --no-tailscale. The --no-tailscale flag is required until the Tailscale identity gate is wired; the daemon refuses to start without it so production deployments can't accidentally run open.

Implements the plan at docs/superpowers/plans/2026-04-28-mairu-acp-bridge.md and the design at docs/superpowers/specs/2026-04-28-mairu-mobile-design.md.

What's in here

Path	Role
`mairu/internal/acpbridge/bridge.go`	`Bridge` type, options, lifecycle
`mairu/internal/acpbridge/session.go`	Subprocess pumps + WS subscriber set + event-id stamping
`mairu/internal/acpbridge/registry.go`	Thread-safe session registry
`mairu/internal/acpbridge/ringbuffer.go`	Bounded ring of stamped server→client frames
`mairu/internal/acpbridge/permission.go`	Permission fan-out + synthetic denial on timeout
`mairu/internal/acpbridge/auth.go`	`PeerAuthorizer` interface + AllowAll + TailscaleAuth
`mairu/internal/acpbridge/agentspec.go`	Default agent exec specs (mairu/claude-code/gemini)
`mairu/internal/acpbridge/ws.go`	WS upgrade + per-conn pumps + Last-Event-ID parsing
`mairu/internal/acpbridge/http.go`	`GET/POST/DELETE /sessions` endpoints
`mairu/internal/cmd/acp_bridge_cmd.go`	Cobra command + flag wiring

All packages have unit tests; e2e against real `mairu acp` lives in `acpbridge/e2e_test.go`.

Test plan

• `go test ./mairu/internal/acpbridge/...` passes (full pre-commit suite green)
• `go test ./mairu/internal/cmd/ -run TestACPBridgeCmd` passes (flag presence + refusal-without-flag)
• Manual smoke on tailnet: `mairu acp-bridge --addr 0.0.0.0:7777 --no-tailscale` from desktop, `wscat` from another tailnet node, verify frames flow

Known deferrals (see plan self-review)

1. `tsnet` embedding. Auth interface is wired; embedding the bridge inside its own tailnet identity is a follow-up. Until then the daemon must run on a host already on the tailnet and bind its tailnet IP, with `--no-tailscale` to bypass the gate.
2. `x-mairu-event-id` placement. Sibling field on the JSON-RPC envelope. Verified non-breaking via the e2e test against `mairu acp`.

Pairs with

#$(date +%s) (`feat/mairu-mobile` — the phone client that consumes this bridge). The mobile branch's e2e auto-skips on branches without acpbridge and will activate once this PR lands.

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