Release 3.3.5: discover nested per-change baseline artifacts

[ncheaz]

The baseline-gate detector at runner-baseline-gate.js only scanned the top level of .ralph/baselines/, which made the nested .ralph/baselines/<change>/<gate>.txt layout invisible. Changes that grouped their per-change baselines under a slug subdirectory (a pattern the BP doc's template implied but did not state explicitly) hit a spurious "no matching baseline artifact found under .ralph/baselines" gate and emitted BLOCKED_HANDOFF before any implementation could start.

This release expands the discovery to all three documented layouts:

• flat unprefixed: .ralph/baselines/<gate>.txt
• flat prefixed: .ralph/baselines/<change>-<gate>.txt
• nested: .ralph/baselines/<change>/<gate>.txt (new)

Implementation:

• lib/mini-ralph/runner-baseline-gate.js: _detectRecordedBaselineGates now also walks one level of subdirectories under .ralph/baselines/, preserving the existing gate-name regex, EXIT footer parse, and sort priority. The returned file field uses the relative path inside .ralph/, so nested matches surface as baselines/<change>/<gate>.txt in agent feedback.
• tests/unit/javascript/mini-ralph-runner.test.js: three new tests cover nested-only discovery, mixed flat+nested coexistence, and a feedback-level regression where a completed pre-flight task plus a nested baseline produce "baseline classification" guidance instead of "missing baseline" handoff.
• OPENSPEC-RALPH-BP.md: documents all three supported layouts and updates the pre-flight task template so future changes can adopt the nested layout without tripping the gate.

Verification:

• jest: 730/730 passing (3 new tests in the baseline-gate describe block; everything else unchanged)
• no lint findings in either touched JS file

No behavioral change for the flat layouts; pure compatibility extension.