fix(docs): pre-emptive Mission Control Ch 3-6 cleanup

Same structural pattern as the Ch 1-2 fixes:

Chapter 2:
- Drop the leftover "submit a log via aster call" tip — superseded by
  the dedicated logs.py / logs.ts submitter

Chapter 3 (Metric Ingestion):
- Explicit "where does this code go" framing for the new wire types,
  the new ingestMetrics method, and the import update
- Restart instructions
- New metrics.py / metrics.ts test submitter so users have something
  runnable that exercises the client-streaming path
- Full updated control.py inside a collapsible <details> at chapter end

Chapter 4 (Agent Sessions):
- Explicit framing for the new AgentSession class as a separate class
  in the same file
- Crucially: shows how to register AgentSession() in the server's
  services=[...] list (the most easily missed step)
- Restart instructions
- Full updated control.py with both services in <details> at chapter end

Chapter 5 (Auth & Capabilities):
- Step 3 reframed as "update your main()" with a stable .aster-identity
  callout (so credentials don't break on restart)
- Step 5: explicit "from control.py output" framing on aster1Qm...
  placeholders, both code blocks and the CLI shell example

Chapter 6 (Cross-Language Interop):
- Reframed cross-language clients as runnable files (ts-agent.ts,
  py-agent.py) with explicit save-and-run instructions
- Address taken from CLI argument instead of hardcoded placeholder,
  matching the Ch 2-3 logs.py / metrics.py pattern

Wording: "canonical version" → "complete version" (less jargon for the
beginner audience the quickstart is aimed at)

Author: emrul