Reconcile dev docs into .context and refresh for v0.1.0 #57

Prep for the first tagged release (v0.1.0). Consolidate the development/progress docs off the repo root and make them reflect current reality.

Moves (repo root -> .context/, lowercase to match convention)

• FEATURE_PARITY.md -> .context/feature_parity.md
• MIGRATION_PLAN.md -> .context/migration_plan.md
• PROGRESS_SUMMARY.md -> .context/progress_summary.md

Keep at root: AGENTS.md, CLAUDE.md, README.md (public entry point).

Refresh to current state

The docs still reference the removed basic AMICATorch/Adam backend, list done features (do_reject, adaptive PDF, Newton ramping) as missing, and describe a non-existent legacy/ dir. Rewrite to reflect: single-model parity (#24), adaptive PDF (#26), multi-model distributional equivalence (#27), best-iterate (#51), degenerate-fit contract (#50), module rename (#34), CI green. Mark the true remaining items: performance benchmark, component sharing, edge-case/stability + save/load tests.

Also

• Update stale references in AGENTS.md (lines 61, 155-157) and .context/plan.md (line 69).
• Update .context/plan.md Priority-1 markers (parity blockers) from in-progress to done.