Declare each provider's defaults once, in plugin.toml

[thesiti92]

Stacked on #62. Settings screens such as Whiteboard's no longer need their own copy of any summarizer default.

Changes

• x-default-by. An option's default can depend on another option's value: { key = "provider", values = { gemini = …, openai = …, anthropic = … } }.
  • The host fills these in during resolution, like plain defaults, so config show and pruning treat them the same.
  • The manifest check rejects one that misses a choice, or whose defaults don't satisfy the option's schema.
• Model follows provider. model defaults per provider: gemini-3.8-flash, gpt-6-luna, claude-haiku-4-5. A CLI user who sets only provider = "openai" gets gpt-6-luna, and a model equal to its provider's default is pruned. default.toml no longer pins model.
• provider_details. This new option, derived the same way, holds each provider's default endpoint, key variables, and whether a custom endpoint may go without a key. The summarizer reads these from its options instead of code.
• x-enum-titles. Provider display names ship in the schema.
• Pruning. The table pass now tries every table, not just empty ones, because an object option is only valid whole.

Verified

• cargo test and cargo xtask test-plugins pass.
• Checked live against OpenAI and Anthropic with only provider set.