docs: add Graft prior-art context comparison

[ctharvey]

Summary

Adds docs/prior-art/graft-beacon-context-comparison.md, a Beacon-focused prior-art analysis of NanoNets/Graft.

The note covers both Beacon layers:

• the current provider-neutral MCP project-understanding contract
• the newer Semantic Object / Adaptive Semantic Object / Orientation context-composition direction

It:

• classifies Graft as the strongest reviewed prior art for Beacon's codebase-orientation and context-efficiency lanes
• maps Graft's deterministic tree-sitter graph, cached meaning layer, lexical + personalized-PageRank retrieval, freshness checks, and bounded source packs into Beacon's object and provider models
• identifies Graft symbol/concept nodes as useful derived Semantic Objects and graft ask output as a strong task-specific View, but not a full consumer/purpose/disclosure/budget-aware Adaptive Semantic Object
• distinguishes Graft's current-code projection from Beacon's broader cross-source, temporal, authority-aware project understanding
• proposes cold, static, Graft push, Graft pull, Beacon push, Beacon pull, adaptive, and Menhir-backed benchmark arms
• defines Time-to-Competence, context-quality, outcome, and composition metrics plus adversarial fixtures
• recommends deterministic-first construction, content-addressed enrichment, graph-ranked lexical retrieval, query-time freshness, bounded substitutive source spans, ranking scopes, and context-value telemetry
• proposes Graft as a possible CodeGraphBeaconProvider adapter or direct benchmark baseline rather than code to duplicate
• narrows Beacon novelty claims and updates planned beacon_find_files, beacon_explain_symbol, temporal blast radius, and Milestone 3 positioning

Scope

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

Validation

• Reviewed against Graft v0.8.2 and public main state on 2026-08-05
• Compared branch against Beacon master: one new prior-art document plus the required .agent/CHANGELOG.md entry
• No tests required for documentation-only changes