fix(cli): scaffold export and ctx set stop hiding the global -o/--output flag

[bmscomp]

Item 1.10 of the CLI performance and stability plan (finding F15, its local-flag rows). Independent of the other Wave 1 PRs. It renames two flags; see Release note.

Why

Two commands define their own --output, which hides the global -o/--output output-format flag. On main (6f4cf61):

• kates test scaffold export quick-load -o json writes the template to a file named json (cli/cmd/scaffold.go:229, StringVarP(…, "output", "o", …)).
• kates ctx set foo --url … -o json fails with unknown shorthand flag: 'o' (cli/cmd/config.go:482). Its own --output, the context's default format, has no shorthand.

Changes

• cli/cmd/scaffold.go: the export filename flag is --file/-f.
• cli/cmd/config.go:
  • The context's default format is --default-output.
  • The help example uses it.
  • ctx set notes on stderr when it gets --output without --default-output. A script written for the old flag now sets this command's output format and leaves the context at table, so it should hear about it.
• cli/cmd/data/doc_entries.yaml (kates docs): the ctx set flag and example.
• The book (10-cli-reference.md, 13-scenario-files.md) and the tutorials (02-all-test-types.md, 06-cicd-integration.md): 8 scaffold export … -o <file> examples become -f <file>.

-o and --output on both commands are now the global flag.

Release note

Renamed flags. kates test scaffold export --output/-o <file> is now --file/-f <file>, and kates ctx set --output <format> is now --default-output <format>. On both commands -o/--output is the global output format flag, as on every other command. kates ctx set warns when given --output without --default-output.

The commit body carries a BREAKING: line, as the repo's earlier breaking changes do.

Tests

Added:

• TestScaffoldExport_OutputIsTheGlobalFlag: scaffold export quick-load --dir <tmp> -o json writes quick-load.yaml and no file named json, and sets the global output mode to json. -f mine.yaml writes mine.yaml.
• TestCtxSet_DefaultOutputLeavesOutputGlobal:
  • ctx set lab --url … --default-output json -o json succeeds and stores json as the context's format, without the note.
  • ctx set staging --url … --output json stores table and prints the note naming --default-output.

On main's flags, the first test finds the template written to json, and the second does not parse.

Plan deviation: the plan's test says -o json "prints JSON". scaffold export has no JSON output, so with -o json it still prints its "Exported" line. JSON output for the commands that lack it is item 5.2, so this PR only frees the flag.

Run locally, passing:

• cd cli && go test ./..., go test -race ./cmd, go vet ./....
• scripts/check-book-style.sh, check-book-contrast.py, check-mermaid.sh, gen-version-matrix.sh --check, gen-chart-table.sh --check.
• golangci-lint run --new-from-rev=origin/main ./...: 0 issues.
• scripts/check-cli-style.sh and scripts/check-cli-compat.sh: OK.
• The backend suite, which cli/** triggers (Build & Test): this PR touches nothing under kates/. ./mvnw test on main's kates/ tree ran 1,254 tests with 0 failures and 0 errors, once for all of Wave 1.

Skipped locally: govulncheck, as go.mod is unchanged; the book's HTML and PDF renders, since the book changes are flags inside code blocks.

CI only: ./mvnw verify with the *IT tests, the Kind and Docker jobs, CodeQL and Trivy.