feat: add ValidateConfig trait and per-service validation

[geoffjay]

Adds a `ValidateConfig` trait to `agentd_common` and implements it across all 12 service configuration sections. Wires `cfg.validate()?` into every service's startup so misconfigurations surface immediately with descriptive error messages before any ports are bound.

Changes:

• `agentd_common::config::ValidateConfig` trait with `fn validate(&self) -> Result<()>`
• `AgentdConfig::validate()` that collects errors from all 12 sections (does not short-circuit)
• `impl ValidateConfig` on all shared config section structs (port > 0, backend enum, URL format, alert thresholds, etc.)
• `impl ValidateConfig` on all 12 service-level config structs in their respective crates (including memory)
• `cfg.validate()?` called in `main()` of: core, orchestrator, wrap, ask, notify, communicate, hook, monitor, mcp, ui, index, memory
• Unit tests for both valid and invalid cases per service

Review feedback addressed:

• Add `ValidateConfig` impls for `EmbeddingConfig` and `LanceConfig` in memory crate; wire validation and shared-config port/host in `memory/main.rs`
• Remove `validate_inner()` indirection in `index/config.rs`; logic now lives directly in `impl ValidateConfig for IndexConfig`
• Make `validate_url()` pub in `agentd-common` for reuse by service crates

Closes #1198

[geoffjay]

This change is part of the following stack:

• feat: add ValidateConfig trait and per-service validation #1212 ◀

_{Change managed by git-spice.}

[geoffjay]

Duplicate of #1206 — created by git-spice confusion over branch HEAD. Closing in favour of the original PR.