Add scoped browser compatibility review guidance for msal-browser

[Copilot]

This PR adds a dedicated browser compatibility review path for msal-browser changes that touch browser Web APIs. It introduces a single compatibility reference and connects it to review instructions so the heavier guidance is consulted only for relevant lib/msal-browser/src/ changes.

• Compatibility reference

  • Added lib/msal-browser/docs/browser-compat-map.md as the central map of browser Web API dependencies.
  • Documents:
    • affected auth flows
    • fallback behavior
    • privacy-mode and cross-browser restrictions
    • upcoming browser changes that may affect MSAL

• Review instructions

  • Added .github/instructions/browser_compat.instructions.md for PRs that introduce, modify, or remove browser Web API usage under lib/msal-browser/src/**.
  • Focuses review on high-risk areas such as:
    • storage partitioning
    • third-party cookie restrictions
    • popup / iframe behavior
    • CSP / COOP / framing constraints

• Documentation workflow

  • Updated .github/instructions/doc_review.instructions.md to require keeping the compatibility map in sync with browser API usage changes.
  • Updated AGENTS.md to point reviewers and agents to the compatibility map and clarify when the instruction applies.

• Scope control

  • The compatibility map is not intended for every review.
  • The instruction is scoped so the map is read only when a lib/msal-browser/src/ change introduces, changes, or removes a browser Web API dependency.

Example of the new review trigger:

When a change to `lib/msal-browser/src/` introduces, modifies, or removes a browser Web API call, review the Browser Compatibility Map for known restrictions and flow dependencies.