feat(cli): implement 'cougr add <piece>' component/system importer

[salazarsebas]

Part of #238

Background

Once a project exists (via cougr new or otherwise), adding a specific capability, session-based auth, a hidden-information mechanic, a standards-layer guard, currently means manually reading an example and copy-pasting the relevant component/system code. This mirrors the distribution model shadcn/ui uses for UI components (copy owned source into your project via a CLI, not an opaque dependency), applied to Cougr's existing example catalog.

Objective

Implement cougr add <piece>, which copies a named, self-contained component/system pair (or standards-layer module) from the example catalog into the current project as editable source, with its mod wiring updated automatically.

References

• Product spec: docs/strategy/06-product-strategy.md (shadcn distribution pattern discussion)
• examples/README.md (existing catalog, the data source this command reads from)
• Candidate first pieces: session-auth (from examples/session_arena), hidden-hand (from examples/hidden_hand), standards/pausable, standards/access-control (from src/standards/ usage in examples/guild_arena)
• shadcn/ui CLI, for the distribution pattern this issue adapts: https://ui.shadcn.com/docs/cli

In scope

1. A catalog manifest (e.g. pieces.toml, embedded in the CLI binary like the new templates) mapping a piece name to its source files, target paths, and any Cargo.toml dependency lines it requires (e.g. adding the testutils feature if the piece includes a harness-based test).
2. cougr add <piece> copies the piece's files into the current project's src/, appends the necessary mod declarations to lib.rs (or the relevant parent module) without clobbering existing code, and prints what was added.
3. Conflict handling: if a target file already exists, the command refuses to overwrite and tells the user exactly what would have been written, rather than silently merging or destroying local changes.
4. cougr add --list prints all available pieces with a one-line description, sourced from the same manifest.
5. At least 3 pieces implemented end-to-end for v1: session-auth, hidden-hand, standards/pausable.

Out of scope

• A remote/community piece registry (Tier 3 in docs/strategy/05-ecosystem-vision.md); all pieces ship embedded in the binary for v1.
• Automatic dependency-version resolution beyond appending a known-good cougr-core feature/version line.

Definition of done

• cougr add --list shows at least 3 pieces.
• cougr add session-auth on a fresh cougr new project results in a project that still builds and passes cargo test after the addition.
• Running cougr add twice for the same piece does not silently overwrite modified files.

[Jessicaayegh]

@Jessicaayegh has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hi maintainer! I’d love to work on this issue if it’s still available. I have a good understanding of the requirements and would be happy to implement the solution. Thanks for considering my request!

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[mrteeednut007-dotcom]

@mrteeednut007-dotcom has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Am interested in working on this issue and can start right away. Please assign it to me.

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[Caneryy]

@Caneryy has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hi there, can I work on this issue?

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Congratulations, @mrteeednut007-dotcom! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

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