Unify secret key validation and fix documentation

- Add SecretKeyValidator as single source of truth for key validation
- Update Secret, Placeholder, ImportService, SecretController to use unified validator
- Fix frontend validation to match backend rules (SecretDialog, RenameDialog)
- Add path traversal protection to TemplateService
- Remove exception type leakage from Router error responses
- Add missing command configs to ArgumentProcessor (diff, verify, export)
- Remove dead cache export code from ExportCommand
- Extract import delay to named constant

Documentation fixes:
- Fix --output to --file flag throughout docs
- Fix keep list to keep show, keep ui to keep server
- Remove false rate limiting claims from WEB_UI.md
- Replace YAML with CSV in export format docs and frontend
- Fix broken links to CLI reference
- Add Secret Key Naming Rules section to CLI reference
- Clarify template placeholder syntax

Beta 4 release

Release v1.0.0-beta.3

Beta 2 release with Web UI

Rewrite README to be concise and accurate

- Made it much briefer (76 lines vs 130)
- Added Interactive Shell as key feature with dedicated section
- Removed encrypted cache section (removed for v1)
- Fixed outdated commands (list -> show, etc)
- Added prominent link to full documentation
- Simplified examples to essentials only
- Made features scannable with emoji icons

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <[email protected]>
Co-Authored-By: Happy <[email protected]>

implement only/except in diff and export

cli ref

this was removed

vault config here

docs