Keep only non-default values in the config file

[thesiti92]

Stacked on #60.

Why. config set wrote every resolved default into ~/.config/diffr/config.toml on the first edit (3506fb2, "Materialize full configuration on edit"). Files therefore kept a frozen copy of the defaults of their time. Later default changes, such as #60's new summary prompt or a newly bundled plugin in order, never reached them. This reverses that decision.

What changes

• Sparse writes. Each config set drops every key whose removal leaves the resolved configuration unchanged, then every table left without values.
  • version stays.
  • A custom plugins.order stays, because removing it would change membership. An order equal to the default goes.
  • Anything with a comment on it is kept, and so is its header.
• Old default prompts. When a file is read, the two earlier default prompts at plugins.bundled.summarize.system_prompt are treated as unset, so existing files pick up the current prompt. Temporary: remove after 2026-12-31.
• One module. Both passes live in src/config/prune.rs and share one path-based remove.
• Editable prompt. The summary prompt shows up in settings screens, and its description links to its default in plugin.toml. A test keeps the link's line numbers accurate.

Verified

• cargo test, cargo xtask test-plugins and the TUI's diffr tests all pass.
• Exhaustive: every setting the schema declares (31) is dropped when set to its default and kept otherwise.
• Generated files: 400 seeded files mixing defaults, other values, comments, explicit headers and old default prompts must keep their meaning, prune idempotently, keep every comment, leave nothing removable and not depend on key order. Breaking pruning three ways (ignoring comments, skipping the table pass, removing without checking meaning) fails a different one of these checks each time.
• Found by these tests: instances set to its default resolved differently from leaving it unset, so it was never pruned. Resolution now fills it in for parallel plugins, the same way it fills in enabled.
• Real-world check: a copy of a real materialized config went from 53 lines to 9 after one config set. It resolves to the same configuration except the prompt, which is now the current default.

Trade-off. A value set to exactly today's default is no longer pinned against future default changes.