v0.7.0rc4

Capture-attention Phase A — activation infrastructure

• New MNEMON_CAPTURE_ATTENTION_ENABLED env-var override on the
  Phase A feature flag. Mirrors the standing-tier pattern
  (MNEMON_STANDING_TIER_ENABLED) — operators can flip activation on
  Fly via flyctl secrets set without a code change + redeploy, and
  the next save picks it up without restarting the server. Accepts
  1/true/yes/on (truthy) or 0/false/no/off (falsy);
  unset / unrecognized falls back to config.CAPTURE_ATTENTION_ENABLED
  (still default False through soak). New
  store._capture_attention_enabled() helper called at request time
  from Store.save and cli attention-status. 5 new tests.
• mnemon attention-status now reports the effective flag value
  with the env-var override applied — a Fly secret flip shows up here
  immediately instead of misleading the operator with the unchanged
  config default.

Calibration fixture privacy hardening

• tests/fixtures/capture_attention_pairs.json is now gitignored.
  PR #153 shipped this path tracked with a placeholder schema —
  intended as a seed, but every operator calibration run overwrites
  it with real vault titles + snippets (personal context, in-flight
  work, etc.) that must not land in a public-repo commit. The
  placeholder schema moves to
  tests/fixtures/capture_attention_pairs.example.json (tracked) so
  future contributors still see the format; the operator output stays
  local-only.

Calibration script fixes (scripts/calibrate_capture_threshold.py)

• VecStore.get(vec_id) -> np.ndarray | None added — mirrors the
  has / delete single-id shape; returns a defensive copy. The
  calibration script's vs.get(vec_id) call site failed on first
  invocation because the method did not exist. 3 new tests (returns
  vector, missing → None, defensive-copy invariant).
• Near-neighbor pair sampling replaces uniform-random. The previous
  random sample across a 2510-memory vault produced pair cosines
  clustered at 0.1-0.4 (clearly-different topics) — operator verdicts
  carried no information about whether the threshold cut should be
  0.80 or 0.85. New sampler picks anchors, takes each one's top
  non-self neighbor above cosine 0.55 (well below the lowest
  calibration threshold so edge-negatives survive), and sorts
  descending so the operator tags high-confidence near-dupes first.
  Verified against the 2026-05-24 prod snapshot: 20-pair sample spans
  cosine 0.751-0.999, entirely in the calibration-relevant range.
  Calibration on that snapshot recommended
  CAPTURE_ATTENTION_THRESHOLD = 0.85 — matches the existing default,
  so no config change needed.