Add ADRs 0001-0006 and README (M0)

Records the decisions the milestone plan already made, with their costs stated rather
than only their benefits:

- 0001 e2ee-trust-model: key hierarchy, the AAD-to-row binding that stops the server
  moving ciphertext between rows, and the four-layer public-key trust story. States
  plainly that revocation is not retroactive, that Connect cannot be a security
  boundary, and that the IdP becomes a key-distribution trust root.
- 0002 minimal-apis: feature modules with explicit registration; capability negotiation
  instead of Asp.Versioning, since client and server upgrade independently when
  self-hosted.
- 0003 sync-protocol: single write path, revision cursors, and the bigserial
  pre-commit sequence gap that silently corrupts sync — plus the per-vault advisory
  lock that fixes it and the test that must prove it.
- 0004 relay-authorization: relay forwards bytes rather than terminating SSH, so
  zero-knowledge survives; server-resolved target IPs in the ticket to defeat DNS
  rebinding; why host addresses must be plaintext when relay is enabled.
- 0005 no-application-layer: why the usual Application/mediator layer earns nothing
  here, with the trigger that would make us revisit it.
- 0006 observability-stack: OTel plus built-in ILogger; liveness excludes dependencies
  so a database blip cannot restart the container and kill live SSH sessions.

Also adds a README covering layout, build, enforced conventions and milestones.

docs/adr/0002-minimal-apis.md

@@ -0,0 +1,50 @@
+# ADR 0002 — Minimal APIs with explicitly registered feature modules
+
+- Status: accepted
+- Date: 2026-07-28
+
+## Context
+
+The API is roughly 60 endpoints: identity and enrollment, a public-key directory, teams,
+vaults and grants, sync, read-only item queries, relay, audit and admin. It is consumed by
+one first-party desktop client.
+
+## Decision
+
+Minimal APIs, grouped into feature modules under `Features/`, registered **explicitly** from
+`Setup/EndpointRegistration.cs`. Typed results (`Results<Ok<T>, ForbidHttpResult,
+ProblemHttpResult>`) throughout, one endpoint per file.
+
+Version with a hard-coded `/api/v1` prefix and **no `Asp.Versioning` package**. Instead ship
+capability negotiation:
+
+- `GET /api/v1/meta` — server version, sync protocol version, crypto spec version, feature
+  flags, `minClientVersion`, and the push caps.
+- `GET /.well-known/dodossh-configuration` — OIDC authority, client id, scopes, relay URL.
+
+## Consequences
+
+- Per-endpoint filters and metadata compose cleanly, and typed results give an accurate
+  OpenAPI document without attribute noise.
+- Explicit registration over reflection scanning: predictable startup cost, trimming-friendly,
+  and every route is greppable. The cost is one line per module, which is worth paying.
+- Minimal APIs' real weakness is that a cross-cutting concern is easy to forget. Two
+  mitigations, both required: group-level `RequireAuthorization`, and an **endpoint-inventory
+  test** that enumerates `EndpointDataSource` and fails the build if any endpoint is absent
+  from an explicit authorization expectations table. A new endpoint therefore cannot be added
+  without making an authorization decision.
+- Capability negotiation over version routing is the right trade for a self-hosted product,
+  where client and server upgrade independently and skew is normal rather than exceptional.
+  `.well-known` also *is* the onboarding story: the user types one server URL and the client
+  discovers the rest. `Asp.Versioning` gets added when a v2 actually exists, not before.
+- The generated OpenAPI document is for third parties and a future CLI. It is emitted at build
+  time to `artifacts/openapi/v1.json`, committed and CI-diffed. **The client's real contract is
+  the `DodoSSH.Contracts` assembly**, guarded by PublicApiAnalyzers so an accidental DTO change
+  is a build error rather than a runtime deserialisation failure on a user's laptop.
+
+### Rejected
+
+- **MVC controllers.** Would work, but bring filter/model-binding machinery this API does not
+  need and blur the one-endpoint-one-file structure that keeps the authz surface reviewable.
+- **Reflection-based endpoint discovery.** Convenient until a route silently disappears
+  because an assembly was not loaded, or trimming removes it.