fix: harden resource limits, path safety, and WASM build (#46)

Security audit follow-up across the workspace:

- webclaw-core: keep the crate WASM-safe. quickjs/rquickjs is now a
  cfg(not(wasm32)) target dependency and the extraction entry point uses
  a direct call on wasm instead of spawning a thread, so it builds and
  runs on wasm32 with or without default features.
- webclaw-core: bound the structured-data scrubber recursion (depth cap)
  so deeply nested attacker JSON-LD / __NEXT_DATA__ cannot exhaust the
  stack.
- webclaw-fetch: stream the response body with a running ceiling so a
  small highly compressed payload cannot inflate to gigabytes in memory;
  redact user:pass@ from proxy URLs before they reach error strings.
- webclaw-cli: contain output filenames inside the chosen directory
  (reject .. / absolute, drop traversal path segments), run --webhook
  URLs through the public-URL SSRF guard, clamp --watch-interval to >=1s,
  and make research slug truncation char-safe.
- webclaw-mcp: char-safe slug truncation (no multibyte slice panic).
- setup.sh / deploy/hetzner.sh: replace eval on read input with
  printf -v, and mask auth key / API token in console output.
- CI: enforce the wasm32 build invariant for webclaw-core.

Tests added for every behavioral change. Bump to 0.6.3 + CHANGELOG.

Cargo.lock

@@ -3219,7 +3219,7 @@ dependencies = [
 
 [[package]]
 name = "webclaw-cli"
-version = "0.6.2"
+version = "0.6.3"
 dependencies = [
  "clap",
  "dotenvy",
@@ -3240,7 +3240,7 @@ dependencies = [
 
 [[package]]
 name = "webclaw-core"
-version = "0.6.2"
+version = "0.6.3"
 dependencies = [
  "ego-tree",
  "once_cell",
@@ -3258,7 +3258,7 @@ dependencies = [
 
 [[package]]
 name = "webclaw-fetch"
-version = "0.6.2"
+version = "0.6.3"
 dependencies = [
  "async-trait",
  "bytes",
@@ -3284,7 +3284,7 @@ dependencies = [
 
 [[package]]
 name = "webclaw-llm"
-version = "0.6.2"
+version = "0.6.3"
 dependencies = [
  "async-trait",
  "reqwest",
@@ -3297,7 +3297,7 @@ dependencies = [
 
 [[package]]
 name = "webclaw-mcp"
-version = "0.6.2"
+version = "0.6.3"
 dependencies = [
  "dirs",
  "dotenvy",
@@ -3317,7 +3317,7 @@ dependencies = [
 
 [[package]]
 name = "webclaw-pdf"
-version = "0.6.2"
+version = "0.6.3"
 dependencies = [
  "pdf-extract",
  "thiserror",
@@ -3326,7 +3326,7 @@ dependencies = [
 
 [[package]]
 name = "webclaw-server"
-version = "0.6.2"
+version = "0.6.3"
 dependencies = [
  "anyhow",
  "axum",