feat: selective field-based config hot-reload

[Ridwannurudeen]

Summary

Implements selective hot-reload for runtime configs as requested in #984. Instead of restarting all orchestrators on every config file change, the system now:

• Reads the raw JSON5 from disk and compares field-by-field against the previous snapshot
• Safe fields (system_prompt_base, system_governance, system_prompt_examples, hertz) are updated in-place on the live RuntimeConfig via setattr() — no orchestrator restart needed
• Structural fields (agent_inputs, agent_actions, cortex_llm, simulators, backgrounds, etc.) still trigger the existing full reload path
• File touched but unchanged — detected and skipped entirely (no reload at all)

Key design decisions

Decision	Rationale
Raw JSON5 deep comparison	Avoids model serialization issues; catches nested changes reliably
hertz validation (must be > 0)	Prevents division-by-zero in the cortex loop
No new dependencies	Uses existing json5 import already in both cortex files
No new files	All logic lives in CortexRuntime and ModeCortexRuntime where _check_config_changes already exists
Both runtimes covered	Single-mode and multi-mode get the same selective reload

Multi-mode specifics

• Top-level safe fields: system_governance, system_prompt_examples
• Per-mode safe fields: system_prompt_base, system_prompt_examples, hertz, description, display_name
• Mode added/removed → full reload
• Transition rules changed → full reload
• Changes to inactive modes update ModeConfig only; changes to the active mode also patch the live RuntimeConfig

Files changed

• src/runtime/single_mode/cortex.py — added _read_raw_config, _detect_changed_fields, _apply_safe_reload; renamed _reload_config → _full_reload; updated _check_config_changes routing logic
• src/runtime/multi_mode/cortex.py — same pattern adapted for ModeSystemConfig + ModeConfig hierarchy
• tests/runtime/single_mode/test_cortex.py — updated existing tests + 11 new selective reload tests
• tests/runtime/multi_mode/test_cortex.py — updated existing tests + 12 new selective reload tests

Test plan

• All 62 runtime cortex tests pass (21 new + 41 existing updated)
• Full suite: 1995 passed (2 pre-existing failures unrelated to this PR)
• Safe-only field changes verified to skip _full_reload
• Unsafe field changes verified to trigger _full_reload
• File-touch-without-content-change verified to skip all reload
• Invalid hertz values (0, negative) rejected with warning
• Per-mode changes on active vs inactive modes handled correctly

Closes #984