Stage 4: TOML integrations — gemini and tabnine migrated to plugin architecture (#2050)

Add TomlIntegration base class in base.py that mirrors MarkdownIntegration:
- Overrides command_filename() for .toml extension
- Extracts description from YAML frontmatter for top-level TOML key
- Renders prompt body in TOML multiline basic strings with escaped backslashes
- Keeps full processed template (including frontmatter) as prompt body
- Byte-for-byte parity with v0.4.4 release ZIP output

Create integrations/gemini/ and integrations/tabnine/ subpackages:
- Config-only __init__.py subclassing TomlIntegration
- Integration-specific update-context scripts (sh + ps1)

Add TomlIntegrationTests mixin with TOML-specific validations:
- Valid TOML parsing, description/prompt keys, {{args}} placeholder
- Setup/teardown, manifest tracking, install/uninstall round-trips
- CLI auto-promote (--ai) and --integration flag tests
- Complete file inventory tests (sh + ps)

Register both in INTEGRATION_REGISTRY; --ai auto-promote works automatically.