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/0009-stable-tool-list-on-upstream-failure.md

@@ -0,0 +1,30 @@
+# ADR-0009: Stable tool list, structured errors on upstream failure
+
+**Status:** Accepted (2026-08-08)
+
+## Context
+
+Bridle proxies several upstreams — the Gitea MCP server, Kanboard, Qdrant — any of which can be
+down while the others are healthy. The design doc did not specify what a connected agent sees in
+that case. Options: keep the tool list stable and error on call; health-check upstreams and omit
+their tools from `tools/list`, notifying via `list_changed`; or refuse to serve at all unless every
+upstream is healthy.
+
+Hiding tools avoids the agent calling something dead, but mutates the tool list mid-session, which
+some MCP clients cache or otherwise handle badly. Failing the whole session lets one flaky
+container block unrelated work.
+
+## Decision
+
+The **tool list stays stable** for the whole session and always reflects the agent's permissions,
+not upstream liveness. A call to a downed upstream returns a **clear structured error** naming the
+upstream and the failure mode.
+
+## Consequences
+
+- No mid-session tool-list churn, so no dependence on client `list_changed` handling.
+- A Qdrant outage does not block Gitea or Kanboard work — failure is isolated per upstream.
+- Agents will occasionally call a tool that cannot currently succeed. The error must be specific
+  enough that the agent reports the real cause rather than inventing a workaround.
+- Upstream health is still tracked and surfaced in the Web Panel; it just does not alter the
+  advertised tool list.