docs(ambient): align init/CLI descriptions with keyword detection; tighten preamble directives

[dean0x]

Summary

Ambient mode gained first-word keyword detection in #235 (prompts starting with implement, explore, research, debug, or plan invoke the matching devflow:<keyword> skill). But the init flow, devflow ambient CLI text, plugin description, and README still described plan-only auto-detection — leaving the docs out of step with the two-path hook behavior.

This PR aligns all user-facing copy and tightens the directives the preamble hook injects.

Changes

Descriptions updated to cover both detection paths (keyword + structured plan):

• src/cli/commands/init.ts — ambient mode prompt note + --ambient flag help
• src/cli/commands/ambient.ts — command description, usage note, enable-success message, and a stale code comment
• src/cli/plugins.ts — ambient plugin description → "Keyword + plan auto-detection"
• README.md — "Ambient intelligence" line now names the keyword start words and structured plans

Injected directives tightened (scripts/hooks/preamble):

• Symmetric structure across both paths (announce briefly → invoke via Skill tool → pass input)
• Explicit input pass-through on both paths (the plan path previously never told the model to pass the plan)
• "Immediately / do not pause to ask whether to proceed" to preserve ambient's zero-friction intent

Notes

• The preamble hook is copied to ~/.devflow/scripts/ at devflow init time, so the directive change takes effect after a re-init.
• The plugin README and preamble header comments were already accurate — no change needed there.

Verification

• npm run build:cli — passes
• bash -n scripts/hooks/preamble — syntax OK