feat(platform): add standalone HTTP Bot adapter

A first-class, vendor-neutral message-platform adapter (http_bot) for
server-to-server integrations (LangBot Space ticketing et al). Drives a
pipeline over plain HTTP with no long-lived connection:

- Inbound: signed POST to the existing unified webhook route /bots/<uuid>,
  carrying a caller-defined session_id mapped to the LangBot launcher id via
  get_launcher_id -> per-session isolation. Preserves pipeline-native N->1
  aggregation for free.
- Outbound: each reply_message / reply_message_chunk becomes one signed
  callback POST to the config-only callback_url, delivered in per-session
  sequence order with retry/backoff -> 1->M multi-reply.
- Sub-paths: /reset (drop a session) and /sync (block for the collapsed reply).
- Auth: symmetric HMAC-SHA256 both directions (timestamp + replay window),
  no JWT/Turnstile, no socket.

Decisions: callback URL is config-only (SSRF closed); reset + sync shipped;
Python + TS reference clients shipped (signing verified byte-identical 3-way).

No core changes: the unified webhook router, aggregator, query pool and
pipeline are untouched. Adapter is auto-discovered from platform/sources/.

Adds:
  src/langbot/pkg/platform/sources/http_bot.{py,yaml,svg}
  src/langbot/pkg/platform/sources/http_bot_signing.py
  docs/platforms/http-bot.md, docs/http-bot-openapi.json
  examples/http-bot/{client.py,client.ts,README.md}
Updates docs/HTTP_BOT_ADAPTER_DESIGN.md (status: implemented).

examples/http-bot/README.md

@@ -0,0 +1,51 @@
+# HTTP Bot Adapter — Reference Clients
+
+Minimal, dependency-light clients for the LangBot **HTTP Bot** platform adapter.
+They show the whole loop: signing a request, pushing a message, and receiving
+multi-part replies on a callback endpoint.
+
+Full guide: [`docs/platforms/http-bot.md`](../../docs/platforms/http-bot.md).
+Machine-readable contract: [`docs/http-bot-openapi.json`](../../docs/http-bot-openapi.json).
+
+## Files
+
+| File | What it is |
+|---|---|
+| `client.py` | Python client + Flask callback receiver (`pip install flask requests`). |
+| `client.ts` | TypeScript/Node 18+ client + callback receiver, **zero deps** (`npx tsx client.ts`). |
+
+Both implement the identical HMAC-SHA256 scheme
+(`sha256=hex(HMAC(secret, "{timestamp}." + body))`) — verified byte-for-byte
+against the adapter.
+
+## Quickstart
+
+```bash
+# Python — Terminal 1: callback receiver (your callback_url target)
+python client.py serve --port 8900 --secret SHARED_SECRET
+
+# Python — Terminal 2: push a message
+python client.py push --url https://your-langbot/bots/<BOT_UUID> \
+    --secret SHARED_SECRET --session ticket-1 --text "hello"
+
+# blocking sync mode
+python client.py sync  --url https://your-langbot/bots/<BOT_UUID> \
+    --secret SHARED_SECRET --session ticket-1 --text "hello"
+
+# reset a session
+python client.py reset --url https://your-langbot/bots/<BOT_UUID> \
+    --secret SHARED_SECRET --session ticket-1
+```
+
+```bash
+# TypeScript (Node 18+)
+npx tsx client.ts serve 8900 SHARED_SECRET
+npx tsx client.ts push  https://your-langbot/bots/<BOT_UUID> SHARED_SECRET ticket-1 "hello"
+```
+
+When the bot replies, the receiver prints each part with its `sequence` and an
+`[FINAL]` marker on the last one — that's the 1→M multi-reply model in action.
+
+> The bot's `callback_url` must be reachable from LangBot. For local testing,
+> expose your receiver with a tunnel (cloudflared / ngrok) and set that URL in
+> the bot config.