feat: default azlin new to private/bastion (no public IP)

[rysweet]

Summary

VMs created with azlin new are now private by default and route through Azure Bastion, matching the Python CLI behavior.

Changes

• Default behavior: azlin new creates VMs without a public IP (bastion-routed)
• Add --public flag to opt into public IP creation
• --no-bastion also opts into public IP (unchanged behavior)
• --private kept as hidden backward-compat flag (now the default, so it is a no-op)
• Validation: --private conflicts with --public/--no-bastion
• Updated credential forwarding docs to reflect bastion-by-default
• Fixed pre-existing test_code_has_workspace_flag_with_default parity test failure

Also done (operational)

• Removed public IP from existing "ia" VM; confirmed bastion routing via azlin list --no-cache showing 10.0.0.6 (Bast)

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Merge readiness

QA-team evidence

• Scenario files: tests/agentic-scenarios/pr-966-bastion-default.yaml, tests/agentic-scenarios/new-command-parity.yaml
• Validation command: gadugi-test validate -f tests/agentic-scenarios/pr-966-bastion-default.yaml
• Validation result: passed (1/1 valid)
• Run command: gadugi-test run -d tests/agentic-scenarios -s pr-966-bastion-default
• Run target: local (debug binary at rust/target/debug/azlin)
• Run result: passed (6/6 steps, 1/1 scenario)
• Parity scenario also run and passed: gadugi-test run -d tests/agentic-scenarios -s new-command-parity (4/4 steps, ALL 22 FLAGS PRESENT)

Documentation

• User-facing docs impact: yes (CLI behavior change)
• Updated docs:
  • docs/how-to/forward-credentials.md — removed --private example, documented bastion-by-default
  • docs/reference/credential-forwarding.md — updated flag table (--private → --public)
• PR description links added: see above

Quality-audit

• Cycle 1: zero findings (correctness, security, logic review)
• Cycle 2: 2 medium findings — stale docs referencing --private as a user-facing flag in docs/how-to/forward-credentials.md:55 and docs/reference/credential-forwarding.md:10. Both fixed in commit ea895bc.
• Cycle 3 (final): zero findings. Verified cycle 2 fixes correct. All 8 changed files reviewed. No remaining --private references in docs. No unrelated changes.
• Final clean cycle: cycle 3 (zero critical/high, zero medium correctness/security findings)
• Fixes followed default-workflow: yes (docs updated, committed, pushed)
• Convergence summary: 3 cycles total. Cycle 2 surfaced doc staleness, fixed immediately. Cycle 3 confirmed clean.

CI

• Checks command: gh pr checks 966
• Result: all green (10 successful, 0 failing, 0 pending)
• Skipped checks: OSSF Scorecard (conditional workflow, skipped on PRs)
• Flaky reruns performed: none
• Real failures fixed: pre-existing test_code_has_workspace_flag_with_default parity test (expected /home/user but actual default is /home/azureuser); fixed in this PR

Scope

• Changed files: rust/crates/azlin-cli/src/lib.rs, rust/crates/azlin-cli/src/parity_tests.rs, rust/crates/azlin/src/cmd_vm.rs, rust/crates/azlin/src/cmd_vm_ops.rs, docs/how-to/forward-credentials.md, docs/reference/credential-forwarding.md, tests/agentic-scenarios/pr-966-bastion-default.yaml, tests/agentic-scenarios/new-command-parity.yaml
• Unrelated changes: none (parity test fix is tightly coupled — it was blocking CI for this PR)

Verdict

• Merge-ready: yes
• Remaining blockers: none