feat: add iFlow CLI support (#1875)

Add `iflow` as a supported AI agent (the key users pass to --ai) across
all relevant configuration files, release scripts, agent context
scripts, and README. Includes consistency tests following the same
pattern as kimi/tabnine additions.

- README: describe `check` generically (git + all AGENT_CONFIG CLI agents)
- README: describe `--ai` with reference to AGENT_CONFIG for full list

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>