Add a copy-paste recipe for computing a claim's contentSha256 #110

Each claim in .gitpin/change-evidence.json needs the SHA-256 of a normalized line slice (lineStart..lineEnd, joined with \n). docs/pr-evidence-gate.md explains the rule but gives no command to compute it, so first-time users hand-craft hashes.

Done when

• docs/pr-evidence-gate.md has a short "Compute the hash" section with a POSIX shell one-liner that reads the slice from a commit with git show <sha>:<path> and hashes it exactly the way the gate does;
• the same recipe is given for PowerShell;
• both recipes are checked against the synthetic fixture: pnpm demo:gate uses docs/protocol.md line 5, and your command must print the same contentSha256 the gate reports (see docs/demos/pr-gate-fail-to-pass.md).

Run pnpm validate before opening the pull request.