refactor: simplify feature gates, make lightweight features always-on

[bug-ops]

Problem

Feature gate proliferation causes combinatorial cfg complexity in main.rs (3 separate tool_executor blocks for mcp/tui/neither) and throughout crate code. Default features that add minimal dep weight create unnecessary cfg noise.

Features to Remove (Make Always-On)

Tier 1: Zero-dep code-only flags

Feature	Rationale
openai	Zero new deps, just a code module
compatible	Depends on openai, zero extra deps
self-learning	Zero deps, one module (evolution)
orchestrator	Zero deps, code-only
router	Zero deps, code-only

Tier 2: Already-paid deps or minimal weight

Feature	Rationale
qdrant	qdrant-client already compiled via zeph-memory unconditionally
vault-age	age crate is small, secret management should always be available
mcp	rmcp is lightweight, eliminates worst cfg complexity in main.rs

Features to Keep Gated

tui, candle, metal/cuda, discord, slack, a2a, index, gateway, daemon, scheduler, otel, mock

Implementation Phases

1. Remove code-only gates from zeph-llm (openai, compatible, orchestrator, router)
2. Remove self-learning gate from zeph-skills
3. Remove qdrant gate from zeph-skills and zeph-mcp
4. Remove vault-age gate from zeph-core
5. Make MCP always-on, collapse 3-way tool_executor in main.rs
6. Update root Cargo.toml features
7. Simplify main.rs (~40 lines removed)
8. Clean up zeph-core forwarding features

Phases 1-4 are independent. Phase 5 depends on 3. Phases 6-8 depend on all prior.

Impact

• ~50+ cfg annotations removed across the workspace
• main.rs shrinks by ~40 lines
• default = [] (everything meaningful is always-on)
• Negligible compile time impact
• Breaking change acceptable pre-1.0

References

• Full plan: .local/plan/feature-gate-simplification.md