Cut 16.6.0 and bump the package version in pyproject.toml, uv.lock,
and CITATION.cff.

Minor: two capabilities and one activated release filter. The agent
gains --distill (-d / -dt): every LLM call of a run (inner agent,
judge, reflexion) is appended as one ChatML record per line to
<state-dir>/distill/records.ndjson, tagged with the article's
pmc_id, for fine-tuning in Unsloth Studio or via
datasets.load_dataset("json"); distill-export converts the corpus
to an on-disk HF dataset behind the new [distill] extra (#136).
uuid_on_collision: merge now recomputes number_of_cases as the exact
union of a build-internal supporting_case_ids carrier when present,
replacing first-wins under-reporting and its scalar-conflict report,
and strips the carrier before write in both dedup paths (916f509).
The release-mode applied_to_treat filter is renamed
drop_low_number_of_cases and now reads the number_of_cases column
the pipeline actually emits; the old case_count column never
survived coercion to the significance phase, so the filter had been
a silent no-op since 16.1.0 — release builds now actually drop
treatment edges backed by fewer than 25 cases (#137). Also repairs
a stale gepa_metric test fixture that CI's missing dspy extra had
masked (2dcfcb3, tests only).

Changelog:
- Versioned the Unreleased section as 16.6.0 - 2026-09-04.
- Added the missing --distill entry under Added with its PR link
  (#136).
- Added the missing drop_low_number_of_cases entry under Fixed with
  its PR link (#137), stating the prior no-op honestly.
- Kept the supporting_case_ids union entry; it was committed
  directly to main and carries no PR link.
- Normalized that entry's double-hyphen to the house em dash.

Docs: docs/configuration/graph.md documents the number_of_cases
exception to merge first-wins (8b669d8); #136 already updated
docs/cli.md and docs/agent.md.

Testing:
- uv run pytest -q -> 1197 passed, 3 skipped (96% coverage)
- uv run ruff check . && uv run ruff format --check . && uv run pyright -> clean / 0 errors
- uv lock --check -> up to date
- uv run mkdocs build --strict -> clean