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/0005-get-rules-tool-with-session-gating.md

@@ -0,0 +1,36 @@
+# ADR-0005: Rules delivered via `get_rules`, enforced by session gating
+
+**Status:** Accepted (2026-08-08)
+
+## Context
+
+Bridle's premise is that the canonical ruleset governs every agent session. But an MCP server can
+only *offer* a `get_rules` tool — it cannot make a model call it. Meanwhile Claude Code auto-loads
+`CLAUDE.md` from disk and Codex auto-loads `AGENTS.md` from disk, unprompted, every session. A tool
+the model may skip is strictly weaker than a file the harness injects.
+
+Three delivery mechanisms were considered:
+
+1. A local sync client (`bridle sync` or a session-start hook) writing rendered files to disk —
+   reliable across agents today, but adds a client-side component the design doc does not have.
+2. The `instructions` field of the MCP `initialize` result — no client install, but whether a given
+   client injects it into the system prompt varies and would need per-agent empirical testing.
+3. The `get_rules` tool alone — matches the design doc, but leaves delivery best-effort.
+
+## Decision
+
+Deliver rules through the **`get_rules(project_id, agent_type)` tool**, and make it reliable by
+**gating the session**: any other tool call before `get_rules` has succeeded returns a structured
+error naming the required call. Per-session state tracks whether rules have been fetched.
+
+## Consequences
+
+- Delivery becomes effectively guaranteed using only the in-band MCP mechanism — no client-side
+  component to install or maintain per agent.
+- Costs one extra round-trip at the start of each session.
+- The error message is part of the product surface and must be unambiguous enough that an agent
+  recovers on the first try.
+- Populating the `initialize` `instructions` field remains available later as belt-and-braces. It
+  reuses the same renderer, requires no client install, and does not change this architecture.
+- Session state now matters: the gate flag must be scoped to the MCP session, and session loss
+  means re-fetching.