feat(config): add ValidateConfig trait and per-service validation

[geoffjay]

feat(config): add ValidateConfig trait and per-service validation

Add a ValidateConfig trait to agentd_common with implementations for all
12 shared config sections. Add AgentdConfig::validate() that collects
errors from every section. Implement ValidateConfig on all service-level
config structs and wire validate() into each service's startup path so
misconfigurations fail early with descriptive error messages.

Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com

fix(config): address review feedback on ValidateConfig PR

• Add impl ValidateConfig for EmbeddingConfig and LanceConfig in
  crates/memory/src/config.rs (blocking gap: memory was the only service
  with no validation at startup)
• Wire validation in memory/main.rs: call validate() on embedding and
  lance configs, load shared config for port/host so port=0 is caught
  before any I/O with a clear error message
• Remove validate_inner() indirection in crates/index/src/config.rs;
  move logic directly into impl ValidateConfig for IndexConfig and
  import the trait in index/main.rs and the test module
• Make validate_url() pub in agentd-common so service crates can reuse
  it without duplicating the http/https check

Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com

[geoffjay]

This change is part of the following stack:

• feat(common): add config module with TOML schema and layered loading #1201
  • test(common): add integration tests for layered config precedence #1202
    • feat(cli): add config init and config show subcommands #1203
      • refactor: migrate services with existing config.rs to shared TOML config #1204
        • feat: add config structs for services with inline env var config #1205
          • feat(config): add ValidateConfig trait and per-service validation #1224 ◀
            • docs: add configuration system documentation #1207

_{Change managed by git-spice.}

[geoffjay]

Duplicate of #1212 — created erroneously by git-spice during restack. Closing in favour of the original PR.