examples: a stock Puppeteer client, two more sites, and the failure paths

[redbotster]

Three new examples alongside the shipped ones, covering territory they don't:

forum-post-puppeteer.mjs — a real puppeteer-core client, not the hand-rolled Agent, registers, logs in, and posts a comment while authenticated (a different site shape and a different activity than the profile-update example). Two things I ran into worth documenting inline for the next person:

• Playwright connects and navigates fine, but context.newCDPSession(page) calls Target.attachToBrowserTarget under the hood in connectOverCDP mode, which isn't on the allowlist — correctly refused, since it's broader than the per-target attach Puppeteer uses. That's why this uses Puppeteer rather than Playwright.
• Element-handle helpers ($eval, .type()) go through DOM.resolveNode, also not allowlisted (DOM.querySelector is, resolveNode isn't). All interaction here goes through page.evaluate(), same as the existing examples.

capture-attr-purchase.mjs — a third site, the --value-attr capture variant (a "copy button" carrying the value in data-clipboard-text rather than element text), and a purchase as the closing activity via the execution-intent stand-in, instead of the weather-API example's read-only call.

failure-modes.mjs — the safety paths, not the happy path. A site whose password policy always rejects the bridge's generated password aborts with site_rejected_password and stores nothing (checked against the vault directly, not just the tool response). A binding holding a wrong password mechanically reports "filled" but the site itself never actually authenticates — worth having explicit, since a control nobody's watched fail is a control nobody's checked.

All three assert the same invariant the shipped examples do: the password/key/token never appears in any tool result, checked programmatically. pnpm typecheck and pnpm build are clean; these are plain .mjs examples like the existing ones, not part of the vitest suite.

Ran all three locally against Chrome on macOS — output:

forum-post-puppeteer.mjs:      OK (register, login, post — password never seen)
capture-attr-purchase.mjs:     OK (register, login, capture via attribute, purchase — nothing leaked)
failure-modes.mjs:             ALL OK (rejected password stores nothing; wrong password never authenticates)