feat(workflows): add --dry-run flag to specify workflow run

[fuleinist]

Summary

Implements issue #2661 — add a --dry-run flag to specify workflow run that previews each step's resolved inputs, prompt, and command invocation without spawning the underlying coding-agent CLI or making any AI calls. Use it to verify what a workflow would dispatch before running for real.

What ships

Engine

• src/specify_cli/workflows/base.py: StepContext gains dry_run: bool = False
• src/specify_cli/workflows/engine.py:
  • WorkflowEngine.execute(..., dry_run=False) propagates the flag to every step
  • Persists dry_run on RunState (save/load) and restores it in resume() so an interrupted dry-run does not silently become a real run
  • dry_run semantics documented in the execute() docstring

Step behavior

• CommandStep (workflows/steps/command/): dry_run=True renders the integration's build_command_invocation(command, args) preview, sets exit_code=0, returns COMPLETED without spawning the CLI
• GateStep (workflows/steps/gate/): dry_run=True returns COMPLETED immediately with a short DRY RUN message; no interactive prompt
• Graceful fallback when an integration does not implement build_command_invocation: preview includes the command name and a one-line note explaining the fallback
• except clause narrowed from bare Exception to (ImportError, AttributeError, KeyError, TypeError, ValueError) so dry-run failures stay debuggable

CLI

• specify workflow run --dry-run (in-module, in __init__.py) — the only place the flag is exposed. After the run, the CLI prints any output['dry_run'] messages so the rendered previews surface in the terminal.

What does not ship (intentional)

Per design review, the specify CLI is scaffolding + workflow orchestration only. The per-stage surface (/speckit.specify, /speckit.plan, ...) belongs to the agent, not the CLI. A previous draft of this PR added specify spec / specify plan preview commands; those have been removed along with the supporting start_at / stop_after step filtering in the engine. Issue #2661's wording has been re-scoped to --dry-run on specify workflow run.

Tests

• Existing dry-run coverage in tests/test_workflows.py
• test_dry_run_persisted_in_run_state: dry_run survives save/load round-trip
• test_resume_restores_dry_run: resume() rebuilds StepContext with the persisted flag so an interrupted dry-run stays a dry-run
• test_dry_run_returns_completed_without_dispatch: CommandStep returns COMPLETED with the rendered preview; no CLI is spawned; uses tmp_path for portability
• test_dry_run_skips_interactive_gate: GateStep short-circuits with a DRY RUN message

Usage

specify workflow run speckit --input spec='Build a kanban board' --dry-run
specify workflow run ./my-workflow.yml --input spec='Photo album app' --dry-run

Closes #2661