Add `--dry-run` flag for mutating CLI commands

[gmoon]

Requirement

REQ-CLI-022 — Dry-run mode for mutating CLI operations

Summary

Mutating commands (add, edit, resolve, refine) should support a --dry-run flag that validates input and prints what would be created or modified without writing to disk. This provides a safety net for agent operations.

Lower priority than schema introspection and skill auto-generation since lattice mutations are local files with git undo available.

Motivation

Inspired by gws --dry-run from gws which validates requests before execution. Their CONTEXT.md instructs agents to always use dry-run for mutating operations first. See SRC-GWS-CLI in the lattice.

Example

$ lattice add requirement --id REQ-NEW-001 --title "Test" --body "Body" \
    --priority P1 --category TEST --dry-run
Would create requirement: REQ-NEW-001
  Title: Test
  Priority: P1
  Category: TEST
  File: .lattice/requirements/test/001-test.yaml
  (dry run — no files written)

Acceptance Criteria

• --dry-run flag accepted on add, edit, resolve, refine commands
• Validates all input (ID format, required fields, edge targets exist)
• Prints what would be created/modified without writing
• --format json works with dry-run for structured preview
• Exit code 0 on valid input, non-zero on validation failure

Lattice

• Derives from: THX-AGENT-NATIVE-TOOLS
• Source: SRC-GWS-CLI
• Priority: P2