docs: repository README, contributor rules, scenario template and authoring guide

README leads with the finding that motivated the repository -- pi emits the
skills section only when a tool named 'read' is active, so Curator's
--no-tools --skill combination made its policy unreachable -- with the measured
before/after table and instructions to reproduce it at zero token cost.

AGENTS.md sets seven rules for anyone changing this repository. The third is the
one that matters most: verify pi's behaviour with a probe rather than inferring it
from the docs. Three claims in the first draft of these documents were wrong and
were only corrected by running one.

The _template scenario carries the isolation defaults and inline warnings at the
places where mistakes have already cost time: --no-tools disabling the skills
mechanism, cwd anchoring .pi discovery, the read override being mandatory rather
than optional, and allowed-tools frontmatter not being enforced in 0.84.3.

scenarios/_template/README.md

@@ -0,0 +1,40 @@
+# Scenario: <name>
+
+<One paragraph: what this agent does and for whom.>
+
+Live service: `<name>.service`.
+Live workspace: `/home/claw/pi-workspaces/<name>`.
+
+## Checklist for a new scenario
+
+1. Copy `scenarios/_template/` to `scenarios/<name>/` and fill every
+   `<PLACEHOLDER>` in `profile.toml`.
+2. Write `.pi/SYSTEM.md` — all six sections. The replacement branch supplies no
+   tool list and no guidelines.
+3. Write `.pi/APPEND_SYSTEM.md` for durable domain responsibilities.
+4. Write the extension. Build it on `shared/extensions/pi-guard-base.ts`:
+   `registerRestrictedRead` (mandatory — it is what keeps skills reachable) and
+   `installGuard`. Give every tool a `promptSnippet`.
+5. Drive it from `shared/lib/py/pi_rpc.py` rather than spawning `pi --print` per
+   message.
+6. Verify before deploying:
+   ```bash
+   scripts/deploy-scenario.sh <name>          # dry run
+   shared/extensions/tests/run-guard-checks.sh
+   ```
+7. Deploy and restart yourself:
+   ```bash
+   scripts/deploy-scenario.sh <name> --apply
+   systemctl --user restart <name>.service
+   ```
+8. Record the conformance row in `docs/isolation-baseline.md`.
+
+## Gotchas that have already cost time
+
+- `--no-tools` disables extension tools too, which removes `read`, which removes
+  the skills section. Never use it. Use `--no-builtin-tools`.
+- `cwd` anchors `.pi` discovery. Launch with `cwd` set to the workspace or
+  `.pi/SYSTEM.md` is silently ignored.
+- `--approve` is required for anything under `.pi/`.
+- A tool without `promptSnippet` is callable but invisible in the prose tool list.
+- Tools must truncate their own output (50 KB / 2000 lines).