Add the process protocol, Linux process service and client

[SaladDay]

Adds the Runtime–process service protocol: the agent host uses it to start and control processes inside a sandbox.

• internal/sandboxprocess/protocol.go defines the protocol: 12 operations, events starting at 0x4001, ProcessSpec/PTYSpec, capabilities, typed failures and strict codecs on internal/sandboxwire. The same package has the generic client and server.
• apps/sandboxio/internal/processservice is the Linux service:
  • explicit launches through a umask trampoline (a private Init() hook);
  • pipes or a PTY (creack/pty);
  • POSIX session scopes, with signal targets, and Cancel escalating TERM→KILL. A signal reaches only processes the service can prove are in the operation's session, tracked by pidfd custody; when it can't prove that, it sends nothing;
  • one SIGCHLD reap loop, processservice.Reap(ctx), owns every child exit, so the service can run as a child subreaper. It needs pidfd support (Linux 5.3 or later), checked at startup;
  • workloads inherit no descriptors other than stdio;
  • replay with acknowledgement and backpressure;
  • operation-ID deduplication with tombstones;
  • owner-loss grace;
  • incarnation checks.
• docs/process-protocol.md documents it, and it has an AGENTS.md boundary row and a repository-map row.

ScopeCgroupV2 is not advertised. RequestIDs strictly increase on a stream, enforced by sandboxwire.RequestSequence.

Checks:

• go test -race -count=3 ./internal/sandboxprocess ./apps/sandboxio/...
• FuzzDecode, 30 s
• go vet, gofmt
• darwin and windows builds of internal/sandboxprocess

Part of the agent-outside-sandbox work, milestone M1, lane L2.

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[blacksmith-sh]

Found 1 test failure on Blacksmith runners:

Failure

Test	View Logs
github.com/MiniMax-AI/OpenAgentCore/services/core/internal/persistence/postgres/ deploymentpg/TestResetStartEscalateAndComplete	View Logs

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