rtk rewrite should consult user TOML filters

Problem

rtk rewrite only checks the built-in Rust registry. It doesn't look at user TOML filters in ~/.config/rtk/filters.toml (or platform equivalent like ~/Library/Application Support/rtk/filters.toml on macOS).

This means the Claude Code hook (rtk-rewrite.sh) can't auto-route commands that only have user-defined TOML filters. The hook delegates to rtk rewrite, gets exit 1, and passes through without filtering.

Reproduction

# Add a user filter for ssh
cat >> ~/Library/Application\ Support/rtk/filters.toml << 'TOML'
[filters.ssh]
description = "Strip SSH connection noise"
match_command = "^ssh\\b"
strip_ansi = true
strip_lines_matching = [
  "^Warning: Permanently added",
  "^Connection to .* closed",
]
TOML

# Filter works when called directly
echo "Warning: Permanently added 'host' to known hosts." | rtk ssh root@host ls
# -> filters correctly

# But rtk rewrite doesn't know about it
rtk rewrite "ssh root@host ls"
# -> exit 1 (no rewrite)

Current workaround

Patch the hook to maintain a manual list of user TOML commands as fallback:

if [ $? -ne 0 ]; then
  USER_TOML_CMDS="ssh|scp|nono|python3"
  FIRST_WORD=$(echo "$CMD" | awk '{print $1}')
  if echo "$FIRST_WORD" | grep -qE "^($USER_TOML_CMDS)$"; then
    REWRITTEN="rtk $CMD"
  else
    exit 0
  fi
fi

Works but requires keeping the hook list in sync with filters.toml manually.

Proposed fix

Have rtk rewrite check user TOML filters as a fallback when the built-in registry has no match. Built-in priority stays the same, it just extends coverage to user-defined commands.

Happy to send a PR if this approach makes sense.