feat(boundary): generate session ID at startup

[SasSwart]

PR Map

1. 👉🏼 feat(boundary): generate session ID at startup #194 (closed — merged into feat(audit): session ID generation, sequence counter, and BoundaryLog wiring #196)
2. feat(audit): add per-session sequence counter #195 (closed — merged into feat(audit): session ID generation, sequence counter, and BoundaryLog wiring #196)
3. feat(audit): session ID generation, sequence counter, and BoundaryLog wiring #196
4. feat(config): session correlation header injection configuration #197
5. feat(proxy): inject session ID and sequence number headers on matching requests #198

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Adds a SessionID field to config.AppConfig and generates a UUIDv4 at the
top of Run in run_linux.go. The ID is logged once at startup. No consumers
yet.

This is the first step of the session correlation RFC: every boundary
invocation gets a stable identity that future work will attach to audit events
and communicate to Bridge.

Changes

• config/config.go: Add SessionID string to AppConfig.
• run/run_linux.go: Generate UUID and log it before dispatching to the
  jail-specific runner.
• go.mod: Promote github.com/google/uuid from indirect to direct
  dependency.

The stub (run_stub.go) is unchanged; it returns an error immediately on
non-Linux platforms, so there is no session to start.

Generated by Coder Agents

[SasSwart]

Closing in favour of #196, which now includes this work.