docs: audit documentation for 1.4 / 1.5 state

Systematic pass over every doc under docs/, the root-level README /
QUICKSTART / CONTRIBUTING, and the PlantUML diagrams.  Memory and docs
had drifted against the code since 1.2 — this catches them up to the
1.4.0 release and the 1.5.0a1 experimental line.

User-facing fixes
- README: fix broken docs/mcp.md link (→ mcp-registry.md); channel
  gateway entry reflects shipped Discord + Slack adapters instead of
  "Slack/Teams planned"; diagrams table mentions both.
- QUICKSTART: docs/*.md relative links were wrong from the repo root;
  wizard version bumped from 0.5.4.
- CONTRIBUTING: add dev extra plus the ruff / mypy / pytest commands
  we actually expect before push.

Reference docs
- architecture.md: 19 tool schemas (was 15), 18 admin tabs (was 14),
  turnstone-bootstrap added to entry-points table, OpenAI provider
  file split (chat/responses/common) documented, 38 SDK event
  dataclasses (was 27 and referenced deleted mq/protocol.py), Slack
  adapter + multi-adapter gateway, plan_agent/task_agent naming,
  governance admin-panel rewrite.
- api-reference.md: full attachment endpoints (POST/GET/content/
  DELETE on /v1/api/workstreams/{ws_id}/attachments) plus the
  multipart mode on POST /v1/api/workstreams/new.
- channels.md: Slack Setup section (Socket Mode app creation, OAuth
  scopes, tokens), Slack CLI/env reference in config table, combined-
  adapter architecture diagram.
- console.md: 18-tab listing (was 13) with Channels/Models/Nodes/TLS
  descriptions and ConfigStore live-edit note.
- docker.md: Slack env vars block; image entry-point list now
  includes turnstone / turnstone-bootstrap.
- sdk.md: attachments methods on the server client, attachments
  example (upload-then-send and at-creation), event count fixed.
- releasing.md: four-track table (stable/1.0, 1.3, 1.4 + main 1.5);
  promotion workflow uses 1.5 / 1.6 numbering.
- settings.md: plan_model / task_model / plan_effort / task_effort
  overrides section.
- governance.md: skill naming (/skill, `skill` field — not /template),
  Prompts/Judge tabs called out.
- security.md: two-token-types wording; src claim values match the
  AuthResult source strings actually emitted.
- mcp-registry.md: SDK package name is @turnstone/sdk.
- tools.md: plan / task renamed to plan_agent / task_agent in the
  section headings and summary table; primary-key table matched.
- design/consistent-hash-ring.md: dead direct-http-transport.md
  pointer redirected to architecture.md.

Diagrams
- 02-package-structure: drop phantom chat.py entry point, add admin
  and bootstrap, add slack/bot.py, rename channels/gateway.py →
  channels/cli.py.
- 16-channel-architecture: Slack is no longer "(future)", add a
  SlackBot class and the slack-bolt Socket Mode edges; wire the new
  bot into ChannelService.  PNGs regenerated from both puml sources.

docs/api-reference.md

@@ -842,6 +842,15 @@ button automatically.
 
 Creates a new workstream. The server supports up to 10 concurrent workstreams.
 
+The endpoint accepts **either** `application/json` (legacy shape) **or**
+`multipart/form-data` when you want to upload attachments at creation
+time.  Multipart requests carry one `meta` field containing the JSON body
+shown below plus zero-or-more `file` parts; each file is validated and
+reserved onto the new workstream's first turn before the dispatch worker
+runs, so queued multimodal turns cannot lose files to racing sends.  If
+validation fails the fresh workstream is rolled back so no orphan rows
+leak.
+
 **Request body:**
 
 ```json
@@ -915,6 +924,100 @@ Status code: `400`
 
 ---
 
+### `POST /v1/api/workstreams/{ws_id}/attachments`
+
+Upload an image or text document and attach it to the caller's next user
+turn on this workstream.
+
+- Images (png/jpeg/gif/webp) are capped at **4 MiB** and validated via
+  magic-byte sniff on upload.
+- Text documents (any `text/*` MIME, allow-listed application MIMEs, or
+  known text extensions) are capped at **512 KiB** and must be UTF-8.
+- Per-(workstream, user) pending cap is **10** attachments.
+
+The attachment moves through three states: `pending → reserved →
+consumed`.  Reservation tokens are threaded through
+`POST /v1/api/send` so a queued multimodal turn cannot lose its file to
+an overlapping send.
+
+Ownership failures are masked as `404` so non-owners cannot enumerate
+workstream existence.
+
+**Content-Type:** `multipart/form-data` with a single `file` field.
+
+**Response (success):** `200`
+
+```json
+{
+  "attachment_id": "att_abc123",
+  "kind": "image",
+  "mime_type": "image/png",
+  "size_bytes": 73240,
+  "filename": "screenshot.png",
+  "state": "pending"
+}
+```
+
+**Errors:**
+
+| Code | Meaning                                                 |
+|------|---------------------------------------------------------|
+| 400  | Missing/invalid form, unsupported MIME, not UTF-8, etc. |
+| 403  | Auth/scope failure                                      |
+| 404  | Workstream not found / not owned by caller              |
+| 409  | Pending-cap reached                                     |
+| 413  | Payload exceeds size cap                                |
+
+---
+
+### `GET /v1/api/workstreams/{ws_id}/attachments`
+
+List the caller's **pending** (unconsumed) attachments for this
+workstream.  Ownership failures are masked as `404`.
+
+**Response:** `200`
+
+```json
+{
+  "attachments": [
+    {
+      "attachment_id": "att_abc123",
+      "kind": "image",
+      "mime_type": "image/png",
+      "size_bytes": 73240,
+      "filename": "screenshot.png",
+      "state": "pending"
+    }
+  ]
+}
+```
+
+---
+
+### `GET /v1/api/workstreams/{ws_id}/attachments/{attachment_id}/content`
+
+Returns the raw bytes of an attachment with its stored `Content-Type`.
+Useful for previewing an image or replaying a document.  Ownership
+failures are masked as `404`.
+
+**Response:** `200` — binary body, original `Content-Type`.
+
+---
+
+### `DELETE /v1/api/workstreams/{ws_id}/attachments/{attachment_id}`
+
+Remove a pending attachment.  Consumed attachments return `404` (they
+are part of a committed conversation turn).  Ownership failures are also
+masked as `404`.
+
+**Response:** `200`
+
+```json
+{"deleted": "att_abc123"}
+```
+
+---
+
 ### `POST /v1/api/workstreams/{ws_id}/delete`
 
 Permanently delete a saved workstream and all its messages from storage.