Preflight: warn instead of hard-fail when missing env vars / az login

[achandmsft]

Fixes #21.

azd up blocked on a clean machine because the preflight hard-failed before azd / Bicep could prompt for missing parameters, and required az login without saying so in the README. Reported by @pamelafox.

What changes

scripts/preflight-claude.ps1 and scripts/preflight-claude.sh now treat the four soft preconditions as warnings instead of hard fails:

Check	Before	After
CLAUDE_ORGANIZATION_NAME empty	exit 1	warn, continue (azd prompts for the Bicep param)
AZURE_LOCATION empty	exit 1	warn, exit 0 (azd prompts)
az not on PATH	exit 2	warn, exit 0 (skip marketplace + quota validation)
az not signed in	exit 2	warn, exit 0 (skip marketplace + quota validation)
Marketplace offer not found	exit 4	unchanged — typos and unreleased SKUs must fail fast
Insufficient quota	exit 6	unchanged — the whole reason the preflight exists (Terraform azapi swallows quota errors into opaque 715-123420)

README.md Prerequisites now calls out az login as a recommended step so the preflight can do its catalog + quota checks. skills/claude-on-foundry/SKILL.md PLAN section now describes the preflight as best-effort.

Verification

Local run with no env vars set:

Preflight: CLAUDE_ORGANIZATION_NAME is not set. azd will prompt for the 'claudeOrganizationName' Bicep parameter at provision time. ...
Preflight: AZURE_LOCATION is not set. azd will prompt at provision time. Skipping marketplace + quota validation ...
exit code: 0

Net effect: a brand-new user can git clone && cd infra-bicep && azd up, get prompted by azd for the values that are normally prompted for, and the deploy proceeds. Users who have set AZURE_LOCATION + run az login still get the full proactive catalog + quota validation.