Legacy plugins.toml removed; curl-based install script ships

Changelog

• 78f3109c90a8da9566f71cd084e3d87c975e81cd Steer agents toward stripe sandbox create in help and agent guidance (#1647)
• cd8a24b5ad0320135217fd21aae369ca1d7ed016 Add curl-based install script for macOS and Linux (#1686)
• 42fedced5b5c8e9192280da2951b84602cd13f3d Add post-install tips for a plugin, starting with directory (#1627)
• ecc609264c8386bc093388febe883a62edef5057 Add pre-installation hint for directory plugin (#1626)
• 7bf2f65d7d67967a4a06fe2e880cf2c7b16196c2 Fix comment in pkd/stripe/client.go (#1687)
• 56560aa4e40ffd6b72e7fd9e997c76c4bab36203 Fix test side effects: filesystem writes, global git config, and network calls (#1692)
• 400fd635b8df69d5e78ac49d7db87ea8d5f90761 Package manager install test improvements (#1696)
• bcec1554872350dad21ea5e289d5ce74c309220e Remove legacy plugins.toml manifest in favor of local plugin metadata (#1689)
• 1bc0f2f1c611d2e28e1b3686acfb526552100ab8 Update go-git to v5.19.1 (#1693)
• f29938f970fdcb5e56c71f8a5eac8cc9b4a7d40f fix(npm): remove bin declarations from platform packages to fix npm10 double-listing (#1695)