Add the sandbox link protocol, relay core and Sandbox I/O bootstrap

[SaladDay]

Adds the Link protocol. Agent-host Runtimes (attach peers) and Sandbox I/O services (serve peers) use it to reach each other through a relay. The relay authenticates both sides, authorizes each service stream and splices it opaquely. It also adds the Provider–Sandbox I/O startup input.

• internal/sandboxlink/protocol.go defines the protocol:
  • the handshake: ServeHello, AttachHello, HelloAccepted;
  • opening a stream: Open/Opened from the attacher, Bind/Bound to the serve peer;
  • the control messages;
  • typed failures;
  • the Authority interface;
  • ServiceNetwork and the egress rules;
  • the file export grant: File binds carry Exports, a list of ExportGrant{ID, ReadOnly}.
• Transport: libp2p/go-yamux/v5 over a binary WebSocket over TLS. Streams keep FIN (CloseWrite) distinct from abort (Reset).
  • The dialers enforce TLS: wss://, or ws:// to a loopback host. Core serves the relay behind the installation's HTTPS ingress.
• internal/sandboxwire gains RequestSequence: RequestIDs strictly increase on a stream, per sender.
• Peers and relay:
  • the serve peer reconnects with the same ServerInstanceID;
  • the attach peer;
  • the relay/ core, which Core embeds in M2. It handles leases, generation staleness, RevokeAttachment and RevokeResource.
• sandboxlinktest is a fixture Authority plus a TLS relay, used by the M1 lanes.
• internal/sandboxbootstrap is the strict startup file: version, link_url, credential, resource. The File service serves the single export world, rooted at the sandbox's /.
• Docs: docs/sandbox-link-protocol.md owns the shared ## Framing section, and docs/sandbox-bootstrap.md covers the startup file. Both have AGENTS.md boundary rows and repository-map rows.

Out of scope: Core's route and database authority (M2), and the oac-sandbox-io binary (a later lane).

Checks:

• go test -race over the link, relay and bootstrap packages; the relay package also ran with -count=20
• FuzzDecode, 30 s
• go vet, gofmt
• darwin and windows builds

Part of the agent-outside-sandbox work, milestone M1, lane L3.

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