Align control plane with MACP spec d31ffb1 and runtime 168ab31

[ajit-zer07]

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PR Description:

Summary

• Remove SessionWatch from reconnect path — the spec no longer includes passive attach in the base protocol. Reconnection
  now uses getSession() polling with exponential backoff until terminal state. Recovery service passes pollOnly: true to skip
  the stream path entirely.
• Fix sender auth mismatch — openSession() now sets sender: '' so the runtime fills it from the authenticated identity,
  preventing rejection of mismatched senders.
• Add INVALID_SESSION_ID error handling — startSession() and send() now map this runtime error code to HTTP 400 instead
  of a blanket 502.
• Contract alignment — capture instructions field from InitializeResponse, advertise listChanged: true for modeRegistry
  and roots capabilities, log runtime instructions during execution.
• Extension mode support — Contribute message type (used by ext.multi_round.v1) maps to proposal.updated in the event
  normalizer. Proto registry registers extension modes with __json__ sentinel for JSON decode fallback.

Test plan

• npx tsc --noEmit — TypeScript compiles clean
• npx jest — all 253 tests pass across 20 suites
• npx eslint "src/**/*.ts" — 0 errors (pre-existing warnings only)
• Manual: start control plane against updated runtime, create a run with Decision mode, verify stream events flow
• Manual: verify stream reconnect uses getSession() polling (not SessionWatch)
• Manual: create a run with ext.multi_round.v1, verify Contribute events normalize correctly
• Manual: verify run recovery without stream handle uses polling path