Update contributors and bump version to 1.6.3

[happyloa]

Motivation

• WordPress.org review indicated the plugin owner aaronhsieh was not listed in the plugin metadata.
• Ensure plugin metadata and header Version stay in sync for release management.
• Update the published Stable tag so the plugin directory reflects the new release.

Description

• Update Contributors in readme.txt from pluginwatchdog to aaronhsieh.
• Bump Stable tag in readme.txt to 1.6.3 and insert a 1.6.3 changelog entry.
• Update the plugin header Version constant in site-add-on-watchdog.php to 1.6.3.
• Changes are limited to readme.txt and site-add-on-watchdog.php and do not alter runtime behavior.

Testing

• No automated tests were run for this metadata-only change.
• Manual verification was limited to inspecting the modified files (readme.txt, site-add-on-watchdog.php).

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Codex Task