feat: establish the public agent Session platform

[xinyao27]

Summary

• establish Spool's canonical positioning as the publishing platform for agent Sessions, with explicit Share, Publish, Link-only, Public, Profile, Discovery, Summary, and Resume language
• rewrite the product, design, repository, package, CLI, Web documentation, SEO, navigation, blog, privacy, and terms around the public Session model
• add publishing and reading/resuming guides plus a documentation source-of-truth map; remove superseded plans and screenshots
• make account deletion fail closed for Hub Sessions, purge owner-scoped D1/R2 data including orphan packs, revoke API tokens, and keep the deletion queue retryable until cleanup succeeds
• add the Hub binding to the scheduled deletion Worker and regression coverage for no-Hub, happy-path, cross-owner, orphan-pack, R2 failure, D1 failure, and deployment-shape cases

Product decisions

• new Shares are Link-only
• Publish is a separate explicit action that makes a Session eligible for Profile and Discovery
• the public homepage is Featured-first during community cold start
• public metadata is author-attributed; Resume creates lineage without mutating the source
• global content-addressed manifests remain retained until a race-safe reference-counting or GC protocol exists

Validation

• pnpm check
• pnpm build
• pnpm --filter @spool/backend exec vp test run tests/deletion-worker.test.ts tests/deletion-worker-deploy.test.ts — 15 passed
• Backend and deletion Worker typechecks
• Web tests — 138 passed; Web typecheck
• CLI tests — 142 passed; CLI typecheck
• formatting and git diff --check

Known unrelated test issue

pnpm test reaches packages/session-view/src/build-output.test.ts and stops because that existing test invokes pnpm exec vite, while the workspace's vite catalog entry now resolves to Vite+ and no vite binary is exposed. The affected package is unchanged by this PR; all adjacent suites and the full production build pass.

No E2E tests were run.