feat(scheduler-sim-repl-verbosity): task 6.1 - RED-phase REPL prompt tests

Adds 2 tests in a new test_repl_prompt_context.c covering the dynamic
prompt's plain-before-first-session baseline and its session/timestamp
content in both step and continue modes (Requirement 5.5).

Required genuinely new test infrastructure: libedit's readline() only
writes its prompt text when both stdin and stdout are real ptys, which
no existing REPL test in this codebase's freopen()-from-file harness
can observe. Built a PTY-based capture technique (openpty(), ECHO
disabled via termios) instead - independently reproduced via a
standalone probe and stress-tested 15x with zero flakiness during
review.

New file rather than extending test_repl_execution_control.c (already
at 948 lines, over the 800-line cap), per this spec's established
split precedent.

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

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

@@ -59,7 +59,7 @@
   - _Boundary: sim_print_decision gate_
 
 - [ ] 6. (P) REPL prompt shows current session and timestamp
-- [ ] 6.1 (P) Write failing tests for the dynamic REPL prompt
+- [x] 6.1 (P) Write failing tests for the dynamic REPL prompt
   - Cover: the prompt is the plain `(sim) ` before any session has begun; once at least one session has run, the prompt includes the current session count and a human-readable timestamp; this holds in both `step` and `continue` modes; the prompt never alters any command's own output
   - Observable: the new test cases fail against today's static `XCE_REPL_PROMPT` macro, which never varies
   - _Requirements: 5.5_