docs: use canonical PostPrism repository links

[zelinewang]

Context

The repository was renamed from postprism-12e78c39 to postprism after the README refresh merged. GitHub redirects the old URL, but the default branch still exposed the old name in active clone instructions, runtime resource links, and the architecture tree.

Changes

• Update both README clone/install paths to https://github.com/zelinewang/postprism.git and cd postprism.
• Rename the README architecture root from postprism-12e78c39/ to postprism/.
• Update CLOUD_CONFIG.githubURL to the canonical repository.
• Update runtime and landing-page setup links to the canonical README #setup anchor; the previous encoded complete-setup-guide anchor no longer exists.
• Update landing-page source links to https://github.com/zelinewang/postprism.

No repository rename, dependency, provider, deployment, or account mutation is included.

Verification

• git fetch origin --prune — based on current origin/main@2c51b2502b674fa193ecb3c916eb65edd532d516
• no postprism-12e78c39 reference remains in tracked content
• canonical repository and README paths verified through the GitHub API
• GitHub Markdown API render — pass
• bun install --frozen-lockfile — 454 installs / 488 packages checked, no changes
• bun run build — 1,769 modules transformed; build pass
• git diff --check origin/main...HEAD — pass
• pre-public sweep — 0 Layer 1-4 blockers; 4 warnings (historical author emails, no README copyright notice, optional CHANGELOG/CONTRIBUTING)

The build retains pre-existing caniuse-lite freshness and >500 kB chunk-size warnings; this link-only change does not alter dependencies or bundles.

Remaining live-host limitation

The hosted Lovable page metadata is still generic: live HTML currently reports title/OG title prism-stream-publish and description/OG description Lovable Generated Project. Updating the Lovable deployment is outside this documentation/link PR. This PR does not claim that hosted metadata is fixed.