Remove auto-generated PIN login and retire PR PIN help branch

[xiaoland]

Objective

Retire the auto-generated PIN login path and remove the PR detail PIN help branch.

Product rules

• Anonymous users may still create DRAFT PRs through natural-language and structured form entry.
• Publishing a DRAFT PR requires an authenticated account.
• Direct creation of an OPEN PR, including event-assisted create, requires an authenticated account.
• WeChat login becomes the shared authenticated path for DRAFT publish and direct OPEN creation.

Scope

• remove auto-generated PIN account bootstrap during PR create and join flows
• remove page-level PIN help presentation from PR detail
• update backend auth guards so publish and direct OPEN create use authenticated identity requirements
• update frontend create and publish flows so they route users into WeChat login when authentication is required
• clean up obsolete PIN-specific copy, telemetry, and compatibility seams

Out of scope

• the ongoing pr-core + #167 + #170 unification slices
• broader identity-domain redesign beyond the PR create and publish path

Why now

Anchor PR has already converged toward the single PR model. Removing the auto-generated PIN branch keeps the PR detail surface aligned with the remaining durable auth model.