feat(feature-dev): add iterative PR workflow to implementation phase

[marwinsteiner]

Summary

• Splits the monolithic Phase 5 (Implementation) into Phase 5: Implementation Planning + Phase 6: Iterative Implementation
• Implementation is broken into 2-5 reviewable increments with PR creation between each
• User confirms the increment plan before implementation starts, and approves each increment before the next begins
• Users who prefer a single PR can opt out of per-increment PRs

Motivation

Addresses #30845 — the feature-dev plugin previously implemented entire features in a single monolithic phase, creating large changesets difficult to code review. Teams need smaller, logical increments that can be reviewed and merged iteratively.

Changes

• plugins/feature-dev/commands/feature-dev.md: Split Phase 5 into Planning + Iterative Implementation, renumber subsequent phases (7→Quality Review, 8→Summary)
• plugins/feature-dev/README.md: Updated documentation to reflect 8-phase workflow with new examples for Phases 5-6
• plugins/feature-dev/.claude-plugin/plugin.json: Bumped version to 1.1.0, updated description

Test plan

• Install the plugin via claude plugin add ./plugins/feature-dev
• Run /feature-dev on a multi-file feature and verify Phase 5 presents an increment plan
• Confirm Phase 6 creates PRs between increments and waits for user approval
• Verify opting out of per-increment PRs still works as a single-PR flow
• Confirm Phase 7 (Quality Review) and Phase 8 (Summary) still function correctly