Define config format v2 and prepare 0.1.13

[sidkmenon]

Diffr now reports config format 2 and prepares release 0.1.13. Whiteboard can migrate saved v1 settings before it starts this binary.

v1 key	v2 key
plugins.bundled.<name>	plugins.shape.bundled.<name>
plugins.external.<name>	plugins.shape.<name> with path
plugins.order	plugins.shape.order; retain bundled., remove external.
classifier	plugins.classify.bundled
Custom classifier.path	plugins.classify.<manifest-name>.path

version = 2

[plugins.shape.bundled.context]
lines = 8

[plugins.classify.bundled]
hide = ["generated", "vendored"]

Custom plugins use path; they do not need an external section. Exactly one classifier runs, and two shape entries with the same manifest name cannot both be enabled.

Config behavior	Result
Explicit v1	Reject before decoding old fields
Omitted version	Use v2 defaults
Config edits	Write version 2; schema, show, and set use the same keys
Custom prompts	Keep saved text; remove the old prompt-default cleanup
Stock entries	Cannot set path
Custom shape names starting with bundled.	Reject before entries can collide

The branch is rebased on #91 and retains its missing-key behavior: summaries turn off and diffr continues. The summarizer component was rebuilt with the new config paths.

Review commits in order:

1. Reject v1.
2. Remove old prompt cleanup.
3. Add the final plugin namespaces and update native consumers.
4. Update TUI fixtures.
5. Update config examples.
6. Prepare 0.1.13.

Deslop removed duplicate source state, tests that mirror defaults, and new documentation prose. Config editing, failed-write safety, plugin identity, schema validation, and UI interaction tests remain.

Validation: Rust tests, TUI tests/typecheck, formatting, typo checks, and Whiteboard candidate integration pass. GitHub checks are tracked on this PR.

Release 0.1.13 is not published. After user review and release, Whiteboard #956 must pin it and check the installed packages before merge.

AI assistance: implemented with OpenAI Codex; reviewed and cleaned up with Claude Code and OpenAI Codex.