docs: add phased implementation plan and architecture decision records

Turns the design doc into an ordered, dependency-correct build plan (phases
0-9) with an exit criterion per phase, and records the twelve architectural
decisions it rests on as ADRs.

Decisions: single Rust binary with routed compose services; Streamable HTTP
only; per-agent bearer tokens with clientInfo as a display hint only; SQLite;
get_rules delivered via session gating; full-document rule delivery; project_id
from day one; enforcement tier scoped to Bridle-mediated actions; stable tool
list on upstream failure; OpenAI-compatible embeddings with model/dim guarding;
pattern RAG gated behind a spike; CLAUDE.md + AGENTS.md as v1 renderer targets.

Deviates from the design doc's original 1-8 ordering by moving the audit log
and multi-project schema into phase 1, building the admin API incrementally
rather than all at the Web Panel phase, downgrading vendor guardrail sourcing
to manual-first, and gating the pattern-example RAG behind a validation spike.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019RZerbsHGF9Ka3bKhCjJ9m

docs/adr/0011-pattern-rag-behind-spike.md

@@ -0,0 +1,28 @@
+# ADR-0011: Pattern-example RAG gated behind a validation spike
+
+**Status:** Accepted (2026-08-08)
+
+## Context
+
+The pattern-example RAG pipeline — clone, chunk, embed, tag, search, refresh — is the largest
+single body of work in the design doc, and it rests on an unvalidated hypothesis: that semantically
+retrieved chunks from an example repository measurably improve an agent's output. Naive chunking
+of source code is known to produce poor retrieval, so doing it well requires AST-aware chunking via
+tree-sitter, which is most of the cost.
+
+## Decision
+
+Insert **phase 5.5, a throwaway spike**, before committing to phase 6. Index one real example repo
+with naive chunking, run roughly ten realistic queries, and judge whether the retrieved chunks
+would actually have improved an agent's output. The spike code is discarded regardless of outcome.
+
+Phase 6 proceeds only if the spike passes. Its deliverable is a written go/no-go recommendation
+with the queries and retrieved chunks as evidence.
+
+## Consequences
+
+- A large speculative build is de-risked for a few days of throwaway work.
+- If the spike fails, the `patterns:` section of the canonical document still exists — names,
+  descriptions, and repository URLs — just without indexed, searchable example code.
+- If it passes, the spike's queries become the seed of phase 6's evaluation set.
+- Phase 6's size estimate stays honest rather than being quietly discovered mid-build.