Remove stale src/harness/ stub agent-runner (superseded by modular refactor + upcoming runner)

[cmungall]

Proposal

Remove the stale src/harness/ stub agent-runner. It was the original sketch of
the skill-invocation harness but was never implemented, and it has been
superseded by the modular src/folds/ work + the agent runner we're about to
build (Step 3).

Why it's dead weight

• All stubs. run_skill1.py, run_skill2.py, run_pipeline.py, oracle.py
  (and the __init__) are NotImplementedError placeholders — nothing invokes
  an LLM (grep -r 'anthropic|openai|claude|codex' src/ → nothing).
• References the pre-refactor world. Built around the old 1200-line monolith
  and the CuratorBundle flow, before PR Refactor expert harmonizer from monolith to modular package #8 (modular harmonize_sm/ +
  expert_harmonizer) and PR build_env + mapping filter for leave-one-cluster-out run environments #6/Fix: port build_env to modular expert_harmonizer (restores main broken by #8) #10 (build_env).
• Actively harmful to import. src/harness/__init__.py eagerly imports all
  runners, one of which has a PEP-604 (X | None) annotation that crashes at
  import on Python 3.9 — this is why build_env was deliberately decoupled into
  src/folds/ rather than living under src/harness/.
• The real runner (Step 3) will be greenfield against the .runs/<cfg>/
  environments and the modular harmonizer, not these stubs.

Remove

src/harness/__init__.py
src/harness/run_skill1.py
src/harness/run_skill2.py
src/harness/run_pipeline.py
src/harness/oracle.py
src/harness/exemplar_selection.py   # (review: exemplar logic may be worth salvaging)

Dependents to handle in the same change

These import src.harness and are themselves NotImplementedError stubs of the
old design — remove them too, or rewrite against the new runner once it exists:

• experiments/phase_a_crossval.py (from src.harness.run_pipeline/run_skill1/run_skill2/oracle import …)
• experiments/phase_b_prospective.py (from src.harness.run_pipeline import …)

Explicitly NOT in scope (keep)

• src/schemas/ — not orphaned; src/metrics/skill1_metrics.py (CuratorBundle)
  and src/metrics/skill2_structural.py (TargetSchema) use it.
• src/metrics/ — the scoring side (Step 4). May need updating for the modular
  world later, but that's a separate review, not part of this removal.

Acceptance criteria

• src/harness/ removed.
• No remaining imports of src.harness anywhere (experiments removed or rewired).
• src/folds/ and src/metrics/ unaffected; full test suite still green.
• If exemplar_selection logic is worth keeping, relocate it (e.g. into the
  new runner) rather than dropping it silently.