Sandboxed agent launch: keep --config, run worktree setup with repo access, scan only real flags

[tps-flint]

Found in a post-merge review of #474, and reproduced.

• The attested re-exec under nono keeps --config, so tps agent start --config <file> works when nono is installed.
• Worktree setup and cleanup can reach the base repository's metadata inside the sandbox, or run before entering it. Today they silently do nothing under a real nono.
• The sandbox-flag scan reads only tps's own arguments: --sandbox <word> inside a message or a wrapped command is not a flag.

Acceptance: a test for each that fails on main. Where the behaviour depends on nono, at least one test runs the real nono profile, where available in CI.