docs: focus the README on convergence and approved outcomes

[shinpr]

Summary

• explain how broad exploration can pull Claude away from the requested outcome
• clarify that the main session owns product scope while specialists provide evidence
• show that documents, tests, and reviews are selected by the change rather than required by default
• distinguish an unclear source of approved technical decisions from a missing-file gate
• follow the plugin installer output before reloading plugins
• link the articles behind the workflow design

Verification

• git diff --check
• pre-commit hooks completed successfully

This is a documentation-only change.