fix: make install.sh use GitHub release artifacts by default

[lookmanrays]

Summary

• make piped install.sh default to GitHub release-bootstrap mode instead of caller-cwd bin/
• preserve explicit/package-local installs based on --bin-dir or script path inside unpacked artifacts
• verify release artifact checksums and manifest before extraction/install
• document the canonical GitHub one-command installer and add acceptance evidence

Verification

• bash -n scripts/install.sh
• sh -n scripts/install.sh
• go test ./cmd/codencer ./internal/release
• go test ./...
• make verify-public-release
• exact raw branch curl installer against v0.3.1 release artifact

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