Add --dry-run flag to run, milestone, and project commands

[snipcodeit]

Description

Implement meaningful --dry-run behavior for the three commands where it matters most. The global --dry-run flag already exists in bin/mgw.cjs (line 43) and is handled for link, but not for run, milestone, or project.

Scope (consolidated from closed issues #129, #130)

mgw run --dry-run (was #130):

• Print pipeline plan: GSD route, worktree path, agents to spawn
• Do not execute: no worktree creation, no agent spawns, no GitHub writes
• Show which issue would be processed and its current pipeline stage

mgw milestone --dry-run (was #130):

• Print issue execution order and dependency graph
• Show which issues are blocked, which are ready
• Do not execute any pipeline steps

mgw project --dry-run (was #129):

• Display full template output: milestone names, issues, dependency graph
• Print summary table instead of making GitHub API calls
• No milestones, issues, or labels created

Notes

The --dry-run flag hook point is already wired (global option in bin/mgw.cjs). Implementation requires reading opts.dryRun in each command handler before spawning agents or making API calls. Works best after lib/pipeline.cjs matures to expose a plan-only mode.

Acceptance Criteria

• mgw run <n> --dry-run prints pipeline plan, makes no changes
• mgw milestone <n> --dry-run prints execution order + dependency graph
• mgw project --dry-run prints template output without creating GitHub resources

Prior Issues

Consolidated from: #129, #130