refactor: remove Bun zero-install; Bun installs normally like Node

[vivek7405]

Problem

Bun zero-install (resolve deps on the fly, no node_modules) is not a reliable foundation and is being removed. Investigation this session established:

• bun cold auto-install is intermittently flaky. A first cold bun run dev can ENOENT on any package, including a latest one (reproduced live: @webjsdev/cli@0.10.27 failed cold on a --saas scaffold, then worked once cached). Usually works, but not reliable for a first-time user's first boot.
• The DB tooling fails cold regardless. webjs db generate/migrate shell out to drizzle-kit (CJS), which require()s drizzle-orm and resolves the wrong version (latest, not the app's pin), giving Export named 'defineRelations' not found. The pin shim cannot fix a CJS require.
• The "no install, instant dev" pitch is therefore only half-delivered on bun, while Node already installs normally and works reliably.

Decision: drop Bun zero-install. Bun apps install normally (bun install), exactly like Node. Bun stays a first-class runtime; it just stops doing the on-the-fly trick. True zero-install (including editor types) is pursued separately on Deno, where it works natively (#669), and is out of scope here.

Design / approach

A forward removal PR off current main (NOT a git reset to before #676, which would lose interleaved unrelated work like #681/#678 and desync repo versions from the already-published npm releases). Keep monotonic versions and all unrelated work; just delete the zero-install code and restore install-based behavior.

Implementation notes (for the implementing agent)

Remove or restore (surgical, since some files mix concerns):

• Scaffold (packages/cli/lib/create.js): make bun create run bun install like Node (drop the skip-install default plus the "Skipped install / resolves on the fly" banner); remove the webjs-bun.mjs zero-install bootstrap generation (Zero-install dev/start on Bun via native auto-install (carve from #669) #675/feat: zero-install dev/start on Bun via auto-install bootstrap #676) and the zero-install comments; revisit the caret-range vs exact dep pinning notes (Revert scaffold @webjsdev/* + drizzle exact pins to semver ranges (after #698) #700/feat: scaffold @webjsdev/* and pg as caret ranges, keep drizzle exact #702) now that install resolves normally.
• Server pin shim: delete packages/server/src/bun-pin-rewrite.js (feat: pin Bun zero-install deps via an onLoad specifier-rewrite from package.json #686/feat: forward inline-safe ranges in the bun zero-install pin rewrite #698/feat: share the server's resolved dep versions with the zero-install importmap #701) and the onLoad pin path in packages/server/src/action-seed-bun.js (KEEP the SSR action-seeding part feat: make SSR action-result seeding work on Bun (#529) #534, surgical), the dev.js integration of the pin transform, and the vendor.js zero-install importmap version-sharing (feat: share the server's resolved dep versions with the zero-install importmap #701).
• CLI: remove packages/cli/lib/import-webjsdev.js (dogfood: bun zero-install bun run dev fails, cli can't resolve @webjsdev/server #709 pin of the cli's @webjsdev/*), packages/cli/lib/runtime-rewrite.js, and the bun zero-install branches in packages/cli/bin/webjs.js.
• MCP: packages/mcp/src/mcp-source.js bun-cache zero-install resolution (fix(mcp): source tool resolves @webjsdev from the Bun cache under zero-install #688).
• Abandon the unmerged zero-install PRs/branches: feat: run webjs db/test/typecheck under bun zero-install (no install) #708 (feat/bun-zeroinstall-tooling) and feat: transparent bun install for non-latest deps under zero-install #720 (feature/bun-transparent-auto-install); close them.
• Docs: agent-docs/runtime.md, docs/app/docs/runtime/page.ts, website/, packages/cli/templates/AGENTS.md, root AGENTS.md, READMEs. Drop the zero-install claims; state Bun installs like Node.
• Versions: bump the touched published packages forward from their current npm versions (cli 0.10.27 up, server 0.8.38 up, core/mcp as needed) so release succeeds; per-package changelogs via the hook.
• Landmines: action-seed-bun.js MUST keep the feat: make SSR action-result seeding work on Bun (#529) #534 seeding; only the pin is removed. Do NOT reset versions backward (npm publish fails on an existing version). The Node path was always install-based and must stay byte-identical.

Acceptance criteria

• bun create webjs my-app runs bun install and produces node_modules (like Node); scaffold messaging no longer claims zero-install.
• No pin-shim / zero-install code remains (grep clean); bun-pin-rewrite.js deleted; action-seed-bun.js keeps seeding.
• A scaffolded Bun app bun run dev/start plus webjs db generate/migrate all work (with node_modules).
• Node path unchanged; tests green (unit + bun matrix + e2e where applicable).
• Docs/scaffold/AGENTS updated; feat: run webjs db/test/typecheck under bun zero-install (no install) #708 and feat: transparent bun install for non-latest deps under zero-install #720 closed.
• Published-package versions bumped forward; changelogs generated.