feat(providers): add DeepMyst as OpenAI-compatible provider

[BahaAbuNojaim]

Summary

• Base branch target (master for all contributions): master
• Problem: DeepMyst, an LLM middleware/routing service with OpenAI-compatible API, is not available as a provider
• Why it matters: Users who use DeepMyst for intelligent model routing and token optimization cannot configure it natively
• What changed: Registered DeepMyst as an OpenAI-compatible provider with Bearer auth, DEEPMYST_API_KEY env var, and aliases deepmyst / deep-myst
• What did not change (scope boundary): No new files, no config schema changes, no changes to existing providers or shared code

Label Snapshot (required)

• Risk label (risk: low|medium|high): risk: low
• Size label (size: XS|S|M|L|XL, auto-managed/read-only): size: XS
• Scope labels: provider
• Module labels: provider: deepmyst
• Contributor tier label: N/A (first contribution)
• If any auto-label is incorrect, note requested correction: N/A

Change Metadata

• Change type (bug|feature|refactor|docs|security|chore): feature
• Primary scope (runtime|provider|channel|memory|security|ci|docs|multi): provider

Linked Issue

• Closes # N/A
• Related # N/A
• Depends on # N/A
• Supersedes # N/A

Supersede Attribution (required when Supersedes # is used)

N/A

Validation Evidence (required)

Commands and result summary:

cargo fmt --all -- --check     # ✅ pass
cargo clippy --all-targets -- -D warnings  # ✅ pass
cargo test factory_deepmyst    # ✅ pass
cargo test resolve_provider_credential_deepmyst  # ✅ pass

• Evidence provided: end-to-end tested with gpt-5.2 and gpt-5.2-optimize models via DeepMyst API (multiple turns, different task types: math, creative writing, translation, technical explanation)
• If any command is intentionally skipped, explain why: N/A

Security Impact (required)

• New permissions/capabilities? No
• New external network calls? No (opt-in via user selecting deepmyst provider)
• Secrets/tokens handling changed? No
• File system access scope changed? No
• If any Yes, describe risk and mitigation: N/A

Privacy and Data Hygiene (required)

• Data-hygiene status: pass
• Redaction/anonymization notes: No personal data in changes
• Neutral wording confirmation: Yes

Compatibility / Migration

• Backward compatible? Yes
• Config/env changes? Yes — new optional DEEPMYST_API_KEY env var (additive only)
• Migration needed? No
• If yes, exact upgrade steps: N/A

i18n Follow-Through (required when docs or user-facing wording changes)

• i18n follow-through triggered? No

Human Verification (required)

What was personally validated beyond CI:

• Verified scenarios: single-message mode with gpt-5.2 and gpt-5.2-optimize models, multiple turns with different prompt types
• Edge cases checked: invalid model name returns proper DeepMyst API error, credential env var resolution
• What was not verified: streaming mode (DeepMyst may or may not support it; falls back gracefully)

Side Effects / Blast Radius (required)

• Affected subsystems/workflows: Provider factory only
• Potential unintended effects: None — purely additive match arm
• Guardrails/monitoring for early detection: Existing provider factory tests cover regression

Agent Collaboration Notes (recommended)

• Agent tools used: Claude Code (Opus 4.6)
• Workflow/plan summary: Explored provider trait system, identified OpenAI-compatible pattern, implemented minimal registration, verified with live API
• Verification focus: Factory wiring, credential resolution, end-to-end API calls
• Confirmation: naming + architecture boundaries followed (AGENTS.md + CONTRIBUTING.md): Yes

Rollback Plan (required)

• Fast rollback command/path: git revert <commit-sha>
• Feature flags or config toggles (if any): None needed — provider is only activated when user explicitly selects deepmyst
• Observable failure symptoms: create_provider("deepmyst", ...) returns error

Risks and Mitigations

• Risk: None — additive-only change to a single file following established pattern

🤖 Generated with Claude Code

[theonlyhennygod]

LGTM. Textbook minimal provider registration — follows existing patterns exactly, includes proper tests, 18 lines total. All CI green.