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/0002-streamable-http-only.md

@@ -0,0 +1,24 @@
+# ADR-0002: Streamable HTTP as the sole MCP transport
+
+**Status:** Accepted (2026-08-08)
+
+## Context
+
+MCP defines stdio and Streamable HTTP transports; an older HTTP+SSE transport is deprecated in
+favor of Streamable HTTP. Bridle is a networked gateway intended to be reachable from agents over
+WireGuard, which rules out stdio as the primary transport. A local stdio shim bridging to the HTTP
+endpoint was considered for clients with weak Streamable HTTP support, as was continuing to serve
+the deprecated SSE transport.
+
+## Decision
+
+Serve **Streamable HTTP only**, at `/mcp`. No stdio shim and no deprecated SSE endpoint at v1.
+
+## Consequences
+
+- Matches the per-agent bearer token model (ADR-0003) directly — the token rides the
+  `Authorization` header.
+- Works over WireGuard without extra components.
+- One transport to implement, test, and secure.
+- If a target agent turns out to have unusable Streamable HTTP support, a `bridle-stdio` bridging
+  binary is the fallback. It is not in scope until a specific agent demonstrably needs it.