fix: Propagates live_print_scope to shell message_callbacks

[EspenAlbert]

Summary

Fixes live_print_scope not reaching message_callbacks because shell streaming runs on pool threads without the caller's contextvars.

What changed

• ShellRun captures contextvars.copy_context() at construction.
• Queue consumer dispatches _on_event inside that snapshot so callbacks (and print_to_live from them) see the caller's scope.
• Docs: run_and_wait / message_callbacks propagation and snapshot semantics.

Reviewer notes

• Not breaking; unset context behaves as before.
• Snapshot at ShellRun creation is intentional: scope is fixed for the run (including retries), not updated mid-flight.
• ShellRunBefore still runs outside the snapshot on the _execute_run thread; only queue-driven callbacks are in scope (matches tfdo stream-handler path).
• Follow-up (tfdo): explicit [run_dir_key] prefixes in apply renderers can stay as defense-in-depth; orchestration may restore live_print_scope for non-renderer paths once this ships.

Test plan

• just pre-commit-ask
• Callback sees prefix under live_print_scope at run_and_wait time
• Nested scope: inner prefix wins at construction

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.29%. Comparing base (87014ed) to head (4df8044).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #20      +/-   ##
==========================================
+ Coverage   86.27%   86.29%   +0.01%     
==========================================
  Files          22       22              
  Lines        2084     2087       +3     
==========================================
+ Hits         1798     1801       +3     
  Misses        286      286              

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