Define azd update strategy

[kristenwomack]

Description

Define the strategy for updating azd version, including manual updates, opt‑in auto‑updates, and channel management (Stable / Insiders). This issue captures goals, non‑goals, constraints, and the phased crawl / walk / run approach agreed in discussion. It serves as the framing document for all follow‑on work in this epic.

In scope

• Document goals (safe, low‑friction, opt‑in, non‑disruptive)
• Document non‑goals (e.g., solving all edge cases up front)
• Define crawl / walk / run sequencing
• Capture explicit constraints (platform installs, failure tolerance)
• Record key open questions and deferred decisions

Out of scope

• Implementation details
• Platform‑specific code changes

Acceptance criteria

• Strategy document exists and is linked from the Epic
• Crawl / walk / run phases are explicitly defined
• Known constraints and risks are documented
• Open questions are captured (not silently resolved)