feat(init): add Kiro agent support

[chenhaitao888]

Summary

Adds rtk init --agent kiro, a new rules-file integration for Kiro (AWS's agentic AI IDE).

Kiro reads .kiro/steering/*.md as steering files (project context). This installs token-savings guidance to .kiro/steering/rtk-rules.md, telling Kiro to prefer rtk <cmd> over raw commands — the same prompt-level approach already used for Antigravity and Kilo Code.

Changes

• src/main.rs — add AgentTarget::Kiro and route --agent kiro (project-scoped, rejects -g)
• src/hooks/init.rs — run_kiro_mode writes/updates .kiro/steering/rtk-rules.md; idempotent and dry-run aware; 2 unit tests
• hooks/kiro/rules.md — the embedded steering file (mirrors hooks/antigravity/rules.md)
• hooks/kiro/README.md — per-agent hook README
• docs + top-level README.md — Kiro added to the supported-agents table, install section, and quick-start

Integration tier

Rules file (prompt-level) — identical mechanism to Antigravity / Kilo Code / Cline / Windsurf. No programmatic hook, since Kiro does not yet expose a tool-interception API.

Test plan

• cargo fmt --all — clean
• cargo clippy --all-targets — 0 warnings
• cargo test --all — all pass, including test_kiro_mode_creates_rules_file and test_kiro_mode_is_idempotent
• cargo install --path ., then verified end-to-end:
  • rtk init --agent kiro creates .kiro/steering/rtk-rules.md
  • --dry-run writes nothing
  • second run is idempotent ("already configured")
  • rtk init -g --agent kiro errors (project-scoped), exit 1
  • kiro listed in rtk init --help

[CLAassistant]

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