WIP: Add VS Code provider controls

[techbrewboss]

Summary

• adds slash-command suggestions in the VS Code chat composer
• adds provider/model picker controls plus a Provider Manager UI for configured profiles
• preserves startup-profile credentials when saving provider changes
• tightens OpenAI custom auth validation so header names require values

Addresses #961

Validation

• npm run lint (vscode-extension/openclaude-vscode)
• bun test src/*.test.js (vscode-extension/openclaude-vscode)
• bun test src/utils/providerValidation.test.ts

[Vasanthdev2004]

Targeted maintainer triage of current head 98c4902d12e328ec45576220c91f94f51ec2c8b4; not a full code review because this is marked WIP/draft and is not mergeable.

Verdict: Needs changes

Blocking issues:

1. GitHub reports the branch as conflicting/dirty with main; please rebase and resolve conflicts first.
2. The current check result is from April 30 and predates a lot of extension/provider-manager work on main, so it is not enough signal for merge readiness anymore.
3. The PR overlaps with sensitive extension provider controls and profile persistence behavior. After rebase, please rerun the VS Code extension tests plus the provider validation tests listed in the PR body and update the PR with the new results.

Once the branch is current and no longer WIP, this can get a real code review against the latest extension architecture.

[techbrewboss]

Doing a fresh pr with updates