feat(web): web workstream — pnpm monorepo, sandbox plugins, web portal SPA + harness web client

[geminixiang]

Summary

Splits the web workstream off main (reset to 95736a1) onto its own branch. Contains the 13 commits that were on main plus in-progress local work, spanning:

• Monorepo migration (chore(workspace): migrate to a pnpm workspace monorepo #132): pnpm workspace layout
• Sandbox backends as plugins (feat(sandbox): sandbox backends as plugins — ADR 0006 + 5 backends extracted #134, ADR 0006): 5 backends extracted to packages
• Web portal (feat(web): web-host route registry + daemon port (S1–S2) #133, feat(web): React SPA — web-client shell + session/admin/vault pages (S3–S5) #135 + follow-ups): web-host route registry + daemon port, React SPA shell, login page / session cookie / OAuth redirect / link-token bypass, conversations sidebar, session dashboard
• WIP (top commit 2a79074): harness-web-contract package replacing daemon-web-bridge; web-client reshaped into client/transport/types; src/web/harness/ host-side modules (adapter, host, http, journal, transcript); ui-admin/ui-session/ui-vault/web-bundle packages deleted; ADR 0007 (full harness web client)

Test plan

• Full suite green at branch tip: 1759 tests (131 files) + workspace package tests, lint/format/knip/build all passing via pre-commit hook

[cloudflare-workers-and-pages]

Deploying mikan with    Cloudflare Pages

Latest commit:	2a79074
Status:	✅  Deploy successful!
Preview URL:	https://0602687b.mikan-1wv.pages.dev
Branch Preview URL:	https://web-workstream.mikan-1wv.pages.dev

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