Harden Termux command validation and shell escaping

[Ridwannurudeen]

Summary\n- centralize Termux shell quoting and input validation in TermuxShell\n- validate slugs, relative paths, and required binary names before command construction\n- use centralized shell quoting for all interpolated command values\n- add unit tests for quoting and validation behavior\n\n## Changes\n- add TermuxShell helper\n- harden TermuxSkillSync command and path handling\n- harden ClawHubTermuxSkillAdapter entrypoint and argument handling\n- add TermuxShellTest coverage\n\n## Notes\n- validation is traversal-safe and control-character-safe while remaining compatibility-oriented\n- local Gradle test execution wasn't possible here because Java/JAVA_HOME is missing