feat: add cougr-cli with cougr add command

[mrteeednut007-dotcom]

feat: cougr add — shadcn-style piece distribution for Cougr projects

Implements cougr add as described in docs/strategy/06-product-strategy.md. Adds a new cougr-cli
workspace member (binary crate, cargo install cougr-cli) that copies named, self-contained
pieces from the example catalog into a user's project as owned, editable source — no opaque
dependency, no manual copy-paste.

What was added

cougr-cli/ — new binary crate wired into the workspace

cougr add --list — prints all available pieces with one-line descriptions and maturity tiers:

session-auth [beta ] Session-key auth: approve once, play frictionlessly...
hidden-hand [experimental] Hidden-card ZK proof flow: init a table, verify deal
proofs...
standards/pausable [stable ] Emergency pause/unpause guard backed by persistent
storage...

cougr add — copies piece files into src/, appends pub mod declarations to lib.rs, and
prints exactly what was written.

Conflict handling — if a target file already exists, the command refuses to overwrite it,
prints the full verbatim content that would have been written (so the user can diff manually),
and exits non-zero. Running cougr add twice never silently clobbers local edits.

Pieces shipped in v1

All piece source is embedded in the binary at compile time via include_str!("../pieces.toml") —
no runtime network calls, no remote registry.

┌──────────────┬──────────┬────────┐
│ Piece │ Maturity │ Source │
├──────────────┼──────────────┼─────────────────────────────────────────────────────────────┤
│ session-auth │ Beta │ examples/session_arena — SessionManager lifecycle (approve │
│ │ │ → tap → renew → fallback) │
├────────────────────┼──────────────┼───────────────────────────────────────────────────────┤
│ hidden-hand │ Experimental │ examples/hidden_hand — init_table / verify_deal via │
│ │ │ circuits::hidden_cards │
├────────────────────┼──────────────┼───────────────────────────────────────────────────────┤
│ standards/pausable │ Stable │ src/standards/pausable.rs — ContractPausable newtype │
│ │ │ over cougr_core::ops::Pausable │
└────────────────────┴──────────────┴───────────────────────────────────────────────────────┘

Testing

11 unit and integration tests, all passing (cargo test -p cougr-cli). Tests cover: file
creation for each piece, lib.rs mod wiring, double-add conflict detection, unknown piece error
messaging, and missing src/ directory detection.

Definition of done

• cougr add --list shows 3 pieces
• cougr add session-auth on a fresh project writes files and updates lib.rs without errors
• Running cougr add twice for the same piece refuses to overwrite and shows a preview

closes #245

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[salazarsebas]

Solid implementation of #245. It currently lives at cougr-cli/ (repo root); once #288 merges and fixes cli/ as the canonical location for the CLI crate, could you move this there and rebase before we merge?

[salazarsebas]

This looks like a solid cougr add implementation, but it's built as a standalone cougr-cli/ crate rather than extending the cli/ crate that's already on main (which has cougr new working). Could you rebase onto main and add the add subcommand there instead, so we don't end up with two competing CLI crates? Let me know if you'd like pointers on the existing cli/ structure.

[mrteeednut007-dotcom]

fixed check

[mrteeednut007-dotcom]

drop points please