feat(client): Vercel protection-bypass header for preview deployments

[nictuku]

Why

Pipeboard's PR previews (https://v0-pipeboard-<hash>-nictukus-projects.vercel.app) are behind Vercel SSO. Any CLI request gets HTTP 401 {"error":{"message":"Protected deployment"}} from the edge, so PIPEBOARD_API_URL could target prod or localhost but never the exact commit under review. That is what kept pipeboard.co#3060 parked: a Microsoft Ads change can only be E2E'd where the production Entra app secrets live, and the only non-prod environment with them is the preview.

What

• New internal/client/headers.go: setCommonHeaders() applies bearer auth, User-Agent and, when VERCEL_AUTOMATION_BYPASS_SECRET is set, x-vercel-protection-bypass.
• All four request sites use it: MCP call, MCP notify, REST Post, and FetchToolsHash (the tools-hash endpoint needs no auth but still needs the bypass on a preview).
• README + CLAUDE.md document the variable.

Behavior is unchanged when the variable is unset. Hosts without deployment protection ignore the header.

Tests

internal/client/headers_test.go: httptest server records headers; asserts the bypass header is present on all four request kinds when configured and absent when not, and that tools-hash stays unauthenticated.

go vet ./... && go test ./...
ok  github.com/pipeboard-co/pipeboard-cli/cmd
ok  github.com/pipeboard-co/pipeboard-cli/internal/client

Follow-ups (not in this PR)

• Enable "Protection Bypass for Automation" on the v0-pipeboard Vercel project and distribute the secret (Doppler dev, agents VM .env). The project currently has none minted.
• Tag a release so Homebrew / the agents VM (still on 0.4.0) pick it up.
• pipeboard.co side (scripts/pr-preview-url.sh, agent prompts, skill docs) is a separate PR.