docs(scheduler-sim-step-verbosity): task 2.1 - validate no out-of-scope changes

Verified (full regression build/test pass, a per-file diff of every
pre-existing REPL test file against the pre-feature baseline,
repl.c's command dispatch confirmed byte-identical, and a two-worktree
step-mode transcript diff against a synthetic fixture with both a
same-context and a differing-context event pair) that: no pre-existing
test assertion was touched, no top-level command changed, and the
transcript diff shows exactly the two same-context preamble lines
removed with every other line - including both events' own content -
byte-identical to the pre-feature baseline. The new cache fields
(last_echo_ts_human/last_echo_session) are read/written exclusively
inside repl_print_event_summary(), confirming the feature is
display-only and cannot affect scheduling decisions or simulation
state (Requirement 2.4). The sole test failure in the full suite
(test_job_rusage_reconciles_mem_reservation_against_usage) is
confirmed pre-existing and unrelated (bug-1758). Read-only validation
task; no production or test code changed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YAWMhfRJFnmmnG8h97yUDd

.kiro/specs/scheduler-sim-step-verbosity/tasks.md

@@ -16,7 +16,7 @@
   - _Boundary: Event Echo Preamble Deduplicator_
 
 - [ ] 2. Validation: regression and real-transcript verification
-- [ ] 2.1 Verify every other REPL command and mode's existing behavior remain unchanged outside the preamble's own deduplication
+- [x] 2.1 Verify every other REPL command and mode's existing behavior remain unchanged outside the preamble's own deduplication
   - Confirm which events are echoed, event ordering, event content, and every other REPL command's behavior and output match their pre-feature baseline exactly except for the preamble line's now-conditional printing; confirm no scheduling decision or simulation state changed as a result of this feature
   - Observable: the existing REPL test suite's pre-existing assertions all still pass, updated only where a specific assertion pinned the old always-print-preamble shape for two events that legitimately share rendered context (each such update documented with which acceptance criterion justifies it), and a step-mode transcript comparison against the pre-feature baseline shows no textual difference outside the preamble line's presence or absence
   - _Requirements: 2.1, 2.3, 2.4_