feat(client)!: non_exhaustive API types with request initializers; Card.external_links and CardFile.uid; external links without a second request; cargo-semver-checks in CI

[dsociative]

Closes #26, closes #24. Stacked on #25 (base branch chore/refresh-card-fixtures; retarget to master once #25 is in).

What changes

1. #[non_exhaustive] on every response model and on KaitenError (FileRef stays a closed enum). Fields remain pub and readable; only struct literals and exhaustive matches outside the crate stop compiling. From here on, adding fields and variants is a minor change — cargo semver-checks classifies the follow-up field additions in this very PR as minor.
2. Request types get initializers — CreateCard::new(board_id, title) (its misleading Default is gone), UpdateCard::default(), CardFilter::default(), then field assignment; all three are #[non_exhaustive], so new inputs are added as Option<T> without touching signatures. The 22 literal constructions in the CLI, MCP and tests were rewritten.
3. Fields the API already sends: Card.external_links, CardFile.uid (the uid key on the classic storage, the string id on the newer one). FileRef::Uid now comes from uid (url parsing is the fallback) and FileRef::matches accepts the wire uid, so every uid shown can be passed back to card file get / download_file.
4. card view --include external_links / get_card include: read the links from the card response instead of a second request #24 done the simple way: card view prints the card's links and MCP get_card returns them from the card response — zero extra requests (verified live with -vv: one request for card view, two with --include comments). --include external_links / include: ["external_links"] stay accepted (CLI/MCP contracts are only extended) but are deprecated like --comments: help annotation, one stderr line, no-op; README lists the deprecated forms, removed only in a CLI major. comments remains opt-in.
5. Review of the old workarounds: derived uid wrappers in the CLI (FileListEntry) and MCP (FileView) replaced by the model field; docs updated (Card, CardFile, ExternalLink, crate docs, README Versioning).
6. CI: cargo semver-checks on pull requests against the PR's base commit — minor by default, major with the breaking label, read live when the job runs (this PR carries it; probe PRs [probe — do not merge] semver gate must fail on a breaking change without the breaking label #28/[probe — do not merge] semver gate (base-commit baseline) must fail without the breaking label #29 showed the red path without it). The check against the last release stays a local step before tagging. Known blind spot documented: a removed derive (CreateCard: Default here) is not reported by cargo-semver-checks 0.50.
7. AGENTS.md (+ CLAUDE.md importing it): the project's working rules, including the compatibility rule for the binary's contracts and the commit gate.

semver

cargo semver-checks check-release -p kaiten-client --baseline-rev v0.5.0 → major, exactly two lints: struct_marked_non_exhaustive (25 structs) and enum_marked_non_exhaustive (KaitenError). With --release-type major: clean. No version bump in this PR — the release decides the number (next 0.y per Cargo's rules).

Migration for downstream users of kaiten-client

• Card { .. } / other model literals → deserialize (serde_json::from_value) or read fields; match err { KaitenError::Api { .. } => .. } needs a _ => arm.
• CreateCard { board_id, title, ..Default::default() } → let mut req = CreateCard::new(board_id, title); req.description = ..; (CreateCard::default() no longer exists). UpdateCard { .. } / CardFilter { .. } → ::default() + assignments.

User-visible behaviour

• card view shows External links: whenever the card has any; --json carries external_links inside the card object.
• card file list --json prints uid for classic-storage files too (the API's own value); MCP FileView.uid likewise.
• MCP get_card includes external_links when non-empty without include.

Verification

299 tests green, clippy/fmt clean; live smoke on dstest (read-only on the frozen fixture card, mutations on the playground card: CreateCard::new / UpdateCard / CardFilter paths, temporary card deleted); a throwaway PR without the label proves the semver job goes red on a breaking change.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 95.93909% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.94%. Comparing base (27a562f) to head (2b8be03).

Files with missing lines	Patch %	Lines
crates/kaiten/src/mcp/mod.rs	94.11%	6 Missing ⚠️
crates/kaiten/src/commands/card.rs	94.73%	2 Missing ⚠️
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Additional details and impacted files

@@                       Coverage Diff                       @@
##           chore/refresh-card-fixtures      #27      +/-   ##
===============================================================
+ Coverage                        86.70%   86.94%   +0.23%     
===============================================================
  Files                               33       33              
  Lines                             4897     4941      +44     
===============================================================
+ Hits                              4246     4296      +50     
+ Misses                             651      645       -6     

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