docs: clarify README — open format, accurate framing, more competitors

[liam-machine]

Summary

• Drop the "three places that don't talk to each other" overclaim — the third bullet (your head) is exactly the thing that talks between the other two. Lead-in is now just "the context lives in three places:" and the bullets carry the punchline.
• Add a paragraph in The solution naming the format as plain YAML and JSON — AI reads it natively, git diffs it cleanly, the canvas is just the human-readable view. Reinforces "no proprietary format, no lock-in" and hands off to the moat paragraph.
• Add Erwin and DbSchema alongside SqlDBM in the competitor list so dedicated modelling-tool users (not just dbdiagram/Lucidchart) see themselves.

Test plan

• Render README on GitHub and skim the problem and solution sections for flow.

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