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.

src/specify_cli/integrations/__init__.py

@@ -53,6 +53,7 @@ def _register_builtins() -> None:
     from .codebuddy import CodebuddyIntegration
     from .copilot import CopilotIntegration
     from .cursor_agent import CursorAgentIntegration
+    from .gemini import GeminiIntegration
     from .iflow import IflowIntegration
     from .junie import JunieIntegration
     from .kilocode import KilocodeIntegration
@@ -63,6 +64,7 @@ def _register_builtins() -> None:
     from .qwen import QwenIntegration
     from .roo import RooIntegration
     from .shai import ShaiIntegration
+    from .tabnine import TabnineIntegration
     from .trae import TraeIntegration
     from .vibe import VibeIntegration
     from .windsurf import WindsurfIntegration
@@ -75,6 +77,7 @@ def _register_builtins() -> None:
     _register(CodebuddyIntegration())
     _register(CopilotIntegration())
     _register(CursorAgentIntegration())
+    _register(GeminiIntegration())
     _register(IflowIntegration())
     _register(JunieIntegration())
     _register(KilocodeIntegration())
@@ -85,6 +88,7 @@ def _register_builtins() -> None:
     _register(QwenIntegration())
     _register(RooIntegration())
     _register(ShaiIntegration())
+    _register(TabnineIntegration())
     _register(TraeIntegration())
     _register(VibeIntegration())
     _register(WindsurfIntegration())