docs: stdlib envelope/replace notes + dev-insecure-key (S2, S3, O1)

- S2: document that the response envelope is statusCode-gated — a returned
  map is only unwrapped into {statusCode, headers, body} when it contains
  statusCode, else the whole map silently becomes the 200 body.
- S3: note that Rhai String.replace() mutates in place and returns (),
  with the sub_string bearer-parsing idiom.
- Document ctx.request fields including the new `method`.
- O1: document PICLOUD_DEV_INSECURE_KEY next to PICLOUD_DEV_MODE in
  CLAUDE.md (the acknowledgement var was previously only in the startup
  error).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -116,6 +116,9 @@ Environment variables consumed by the `picloud` binary:
 | `PICLOUD_BIND` | `0.0.0.0:8080` | HTTP listen address. Port 8080 is owned by another process on this host — override locally. |
 | `PICLOUD_MAX_CONCURRENT_EXECUTIONS` | `32` | Global concurrency cap on data-plane script executions. Overflow returns HTTP 503 with `Retry-After: 1` immediately (no queue). |
 | `DATABASE_URL` | — | Required. Postgres connection string. |
+| `PICLOUD_SECRET_KEY` | — | Master encryption key (base64). Required at startup unless dev mode is acknowledged (below). |
+| `PICLOUD_DEV_MODE` | `false` | `true` enables local-dev conveniences. Without `PICLOUD_SECRET_KEY` it ALSO requires the acknowledgement var below — `PICLOUD_DEV_MODE=true` alone aborts at startup. |
+| `PICLOUD_DEV_INSECURE_KEY` | — | Set to the literal `i-understand-this-is-insecure` to let dev mode boot without `PICLOUD_SECRET_KEY`, using a deterministic, world-known dev master key. Never set in production — it would encrypt everything with a public value. |
 | `PICLOUD_DB_MAX_CONNECTIONS` | `32` | Postgres pool size. Matched to `PICLOUD_MAX_CONCURRENT_EXECUTIONS` so the data plane can't starve background workers. |
 | `PICLOUD_SESSION_TTL_HOURS` | `24` | Sliding-window session lifetime. |
 | `PICLOUD_SANDBOX_MAX_*` | conservative defaults | Per-knob admin ceilings on Rhai sandbox overrides. See `manager-core::sandbox::SandboxCeiling`. |