feat(cli): contextual fallback hints on provider failure

[anhermon]

Thinking Path

1. Anvil project → CLI binary (crates/cli)
2. Provider selection in run.rs and eval.rs commands
3. Error handling path when provider fails (creation or runtime)
4. HarnessError variants: Provider, Config, Api
5. User sees raw error with no guidance on what to try next
6. Need: contextual hint appended to error message based on which backend failed
7. Solution: provider_fallback_hint() function in commands/mod.rs, applied at error boundaries

What Changed

• Added provider_fallback_hint(backend) in crates/cli/src/commands/mod.rs — returns a contextual suggestion string based on the failing provider
• run.rs: wrapped provider creation error, streaming error, and agent run error with fallback hints
• eval.rs: wrapped provider creation error and agent run error with fallback hints
• Added 4 unit tests covering cc, claude-code, claude, and echo hint behavior

Verification

• cargo test --workspace — all tests pass (14 CLI tests including 4 new)
• cargo clippy -- -D warnings — clean
• cargo fmt — applied
• Manual verification: when cc fails → "Hint: Try --provider claude with ANTHROPIC_API_KEY set"
• Manual verification: when claude fails → "Hint: Set ANTHROPIC_API_KEY or try --provider echo for testing"
• Echo provider returns no hint (no fallback needed)

Risks

Low risk — only appends text to existing error messages. No behavioral changes to success paths. No new dependencies.

Checklist

• cargo test passes locally
• cargo clippy -- -D warnings is clean
• cargo fmt has been run
• New behaviour has a test (using EchoProvider where possible)
• Commit messages follow Conventional Commits
• The PR description explains why, not just what

Closes ANGA-577

[anhermon]

CTO Review — Approve

Clean contextual fallback hints. 4 unit tests cover all match branches. Low risk — only error paths touched.

Suggestions for follow-up:

1. Extract repeated .map_err closure into fn with_hint(e, backend) helper
2. Catch-all _ arm assumes Anthropic — add comment or use explicit match arms for future providers

Non-blocking. ✅