feat: add --output toon (token-efficient output format)

[apgiorgi]

Summary

Adds toon as an opt-in value for --output, emitting TOON (Token-Oriented Object Notation) — a compact, lossless format that uses ~40% fewer tokens than JSON for list-shaped data, useful when an LLM agent drives the CLI. table remains the default.

• New dciToonContentType mirrors the existing dciTableContentType: toJSONSafe → toon.Marshal, degrading to indented JSON on any encode error.
• Registered with restish under the short name toon; wired into the --output validation switch, help text, and error message.
• Adds github.com/toon-format/toon-go (pure-Go, no cgo — keeps cross-compilation portable).
• README documents the format.

Implements the TOON slice of #9. Schema/projection/truncation/limit work remains open on #9 and is intentionally out of scope. Coordination note left on #21 (agent-mode) so it can adopt toon as the agent default.

Design + plan: docs/superpowers/specs/2026-06-08-toon-output-design.md, docs/superpowers/plans/2026-06-08-toon-output.md.

Test Plan

• go build ./... && go vet ./... && go test ./... all green
• dciToonContentType.Marshal unit tests: list (tabular), object (real TOON, not JSON fallback), scalars
• TestOutputFlagValidation drives real cobra Execute — accepts table/json/yaml/auto/toon, rejects unknown
• Manual: dci list-budgets --output toon renders TOON against the live API; --output bogus errors with the supported list
• CGO_ENABLED=0 confirms toon-go introduces no cgo requirement

[Copilot]

Pull request overview

Adds an opt-in --output toon format to the CLI, using the TOON (Token-Oriented Object Notation) encoder for more token-efficient, agent-friendly output while keeping table as the default.

Changes:

• Add toon to --output flag help + validation and register a new dciToonContentType formatter.
• Add unit tests covering TOON marshaling and --output flag validation.
• Add the github.com/toon-format/toon-go dependency and document TOON in README + internal design/plan docs.

Reviewed changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
README.md	Documents --output toon and links to the TOON format site.
main.go	Registers toon output, validates the new flag value, and implements dciToonContentType marshaling with JSON fallback.
main_test.go	Adds tests for TOON marshaling and for --output flag validation behavior via Cobra execution.
go.mod	Adds github.com/toon-format/toon-go dependency.
go.sum	Adds checksums for the new TOON dependency.
docs/superpowers/specs/2026-06-08-toon-output-design.md	Adds design spec for the TOON output slice of #9.
docs/superpowers/plans/2026-06-08-toon-output.md	Adds an implementation plan for TOON output.

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[apgiorgi]

Addressed the Copilot review:

• main_test.go — list test could pass on JSON fallback ✅ Fixed in f7173c7. Added a strings.HasPrefix(s, "{") guard so TestToonMarshalListProducesTabularOutput fails if it silently falls back to indented JSON (mirrors the object test's check).
• The 3 comments on docs/superpowers/specs/... and docs/superpowers/plans/... — moot. Those scaffolding files were removed from this branch (force-pushed); the design/scope notes now live as a comment on Token-efficient output format #9 instead. Copilot reviewed the pre-cleanup revision.

[apgiorgi]

LGTM