docs(readme): the consumer shape; lib is the manual for what is shared

[jshvn]

What changes

The README takes the consumer shape used across the org: the lede, How to use, How it works (the pipeline drawn, this mirror's verbs dashed), Want your own? (fork, storage, secrets, the zone, prove it), Operating it, Reference. Everything the mirrors share links to katoptra/lib's README (katoptra/lib#5) rather than being repeated: the vault, the toolbox verbs, the workflows, the state.

• CONTRIBUTING.md follows lib's pattern: the org's rules apply, plus what this repository adds.
• SECURITY.md is the guarantee list and the reporting link.
• CLAUDE.md drops the restatement of what the mirror owns and the secrets bullet; it keeps the must-knows and the hazards and points at the README.
• op.env and docs/reference.md no longer name the org's vault or dispatcher; a fork reads the same sentence and it is true.

No verb changes; render.txt is untouched.

Storage and operations

None.

Test plan

• task check passes (the check workflow)
• The rendered README's diagram draws and its lib links resolve once docs(readme): the manual, with every stage drawn lib#5 merges