Refactor pr-build.yml: eliminate instructions.md temp file

[jongio]

Problem

The pr-build.yml workflow uses an intermediate instructions.md file to pass content between the "Generate installation instructions" step (run: step) and the "Comment on PR" step (actions/github-script). This is fragile because:

1. defaults.run.working-directory: cli makes run: steps write to cli/instructions.md
2. uses: steps (like actions/github-script) ignore defaults.run.working-directory and execute from repo root
3. The path mismatch caused the PR comment step to fail with ENOENT: no such file or directory

This same bug existed in azd-copilot (opposite direction) and was caught during PR #14 diagnosis.

Proposed Solution

Eliminate the instructions.md temp file entirely by inlining the instruction content generation in the actions/github-script step. The script already has access to all step outputs (steps.version.outputs.version, steps.pr.outputs.number, etc.) and can construct the markdown string directly in JavaScript.

This removes the fragile file handoff between steps and makes the workflow immune to working-directory mismatches.

Scope

• Refactor azd-rest pr-build.yml
• Apply same pattern to azd-copilot, azd-app, azd-exec for consistency
• Test by opening a PR and verifying the comment appears

Workaround

The immediate path fix (readFileSync('cli/instructions.md')) has been applied in the current codebase.