feat(errors): add structured browser error taxonomy

[pt-act]

Summary

Adds a typed, serializable error taxonomy for browser failures and routes CDP-eval and navigation errors through a single mapper. Each error carries a stable kind/code, a redacted context (URL query/fragment secrets stripped), and a recoveryHint. ElementResolutionError moves into the shared ego-errors.ts so every driver shares one definition. Smallest useful change: purely additive — errors still throw, they are now typed and consistent.

Related issue

No tracked issue — this is the foundation for a small series of reliability/ergonomics improvements to the helper surface (session resilience, lifecycle hooks, semantic actions), each landing as its own focused PR.

Changes

• src/ego-errors.ts (new) — EgoError base + ElementResolutionError, NavigationTimeoutError, ConnectionLostError, TimeoutError, DialogBlockingError; mapCdpError(raw, ctx) with clear precedence; redactUrl; create*Error factories; toJSON() for serializable, secret-free errors.
• src/element-resolver.ts, src/driver/locator.ts, src/driver/waits.ts — import ElementResolutionError from ego-errors (moved, single source of truth).
• src/cdp-eval.ts — drop the duplicate local TimeoutError; typed operation-timeout.
• src/driver/load.ts, src/driver/nav.ts — throw NavigationTimeoutError on the real timeout path.
• src/helpers.ts — expose mapCdpError on the helper surface.
• package/ego-browser/README.md — one Source-layout line documenting ego-errors.ts.

Verification

npm test                      # 316 pass / 0 fail (build + tsc --noEmit + node --test)
npm run validate:site-skills  # site skills ok

Targeted tests cover URL redaction (both message and toJSON), mapCdpError precedence, and the real navigation-timeout path via a fake clock (src/ego-errors.test.mjs, src/driver/nav.test.mjs).

Impact

• Public helper API or behavior
• Agent skill or instructions
• Site learning
• Installation or update flow
• Build, CI, or release process
• Documentation only
• No externally visible impact

Additive and backward-compatible: no public helper removed; drivers now throw typed errors instead of ad-hoc ones. ElementResolutionError keeps its transient/permanent classification that wait loops rely on. No new runtime dependencies.