docs: add Atlaso Ambient Memory prior art

[ctharvey]

Summary

Adds docs/prior-art/atlaso-ambient-memory-comparison.md, a Beacon-focused comparison of Atlaso's automatic recall and closed Ambient Memory product.

The note:

• treats Atlaso as direct product prior art for automatic session-start Orientation, cross-tool continuity, conflict-aware memory bullets, and background enrichment
• separates the publicly inspectable recalled-memory block from the closed Ambient Memory composer
• reconstructs Cursor's observable session-start path: broad fixed seed, project-scoped hosted recall, recent-memory fallback, deduplication, conflict markers, and alwaysApply rules injection
• maps Atlaso memory, conflict state, and Ambient Memory into Beacon Views, Semantic Objects, Adaptive Semantic Objects, and Orientation
• distinguishes Atlaso's memory-first returning-user continuity from Beacon's provider-neutral composition across purpose, docs, code, Git, decisions, benchmarks, guardrails, and Menhir
• adds delivery, freshness, scope-parity, source-authority, disclosure, context-budget, and injection-safety requirements
• proposes cold, recent-only, Atlaso-like generic, static Beacon, Graft, task-shaped, Menhir-backed, and adaptive push/pull benchmark arms
• narrows Beacon novelty claims and positions Ambient Memory as one useful provider/policy rather than the whole project-understanding contract

Stack note

This PR is intentionally based on docs/graft-prior-art-20260805 / PR #2 because both documentation changes update .agent/CHANGELOG.md. Relative to that branch, this PR contains only the Atlaso prior-art document and its changelog entry. Once PR #2 lands, this PR can be retargeted to master without content changes.

Scope

Documentation only. No runtime, schema, tool-contract, or dependency changes.

Validation

• compared against the Graft prior-art branch: one new Atlaso document plus seven changelog lines
• source links point to Atlaso's official product/research pages and public connector repositories
• no tests required for documentation-only changes