docs: ship bootstrap prompt and document first-run sequence

Pi only acts when prompted. New repo users (and the maintainer's future
self) shouldn't have to invent the kickoff message — pin it.

- prompts/bootstrap.md: the canonical first-run message, with rationale
  for each clause and guidance for shorter subsequent prompts
- README quickstart: new "Send the first message" subsection that quotes
  the bootstrap prompt verbatim and explains what the agent does next

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

prompts/bootstrap.md

@@ -0,0 +1,31 @@
+---
+name: bootstrap
+description: First-run prompt to send to Pi after `pi /login` and `cp .env.example .env`. Kicks the agent into building its own mineflayer bridge per AGENTS.md.
+when_to_use: First time launching the agent in a fresh checkout, or whenever you want to reset and re-run the bootstrap loop from scratch.
+---
+
+# Bootstrap prompt
+
+Paste this as the very first message in a new `pi` session:
+
+```
+You're awake. Read AGENTS.md and the repo's current state, then begin executing "First objective — bootstrap your own body" from AGENTS.md. Walk me through each step before you run it the first time — I want to see which Pi tooling (extensions API, skill API, plain bash, etc.) you choose for the mineflayer bridge.
+```
+
+## Why this shape
+
+- **"You're awake"** — soft cue that this is the beginning of a session, not a one-shot question.
+- **"Read AGENTS.md and the repo's current state"** — Pi auto-loads AGENTS.md, but a second explicit read keeps the agent honest about which version it's working from. It also forces a quick scan of `package.json` / `.env.example` / existing `skills/` and `extensions/`.
+- **"Begin executing First objective"** — direct hand-off to the procedural list in AGENTS.md. No need to re-state the steps here.
+- **"Walk me through each step before you run it the first time"** — keeps a human in the loop for the first bootstrap. After the loop is stable you can drop this clause and let the agent run unattended.
+- **"…which Pi tooling you choose"** — surfaces an architectural decision (extension vs skill vs bash) instead of letting Pi make it silently.
+
+## After the first run
+
+Once the bridge is working and `skills/server-onboarding.md` is written, the next session prompt can be much shorter, e.g.:
+
+```
+Resume. Check the server's online, log in if needed, and report status.
+```
+
+Or schedule a tick prompt (see `prompts/tick.md` once that exists).