feat: support loading config from remote URL

[neilkuan]

Closes #80

What

Auto-detect URL in the existing positional config argument. If it starts with http:// or https://, fetch the TOML content via HTTP GET; otherwise treat it as a local file path (existing behavior unchanged).

# local (unchanged)
agent-broker config.toml

# remote (new — auto-detected)
agent-broker https://example.com/config.toml

Implementation

• Added reqwest dependency with rustls-tls backend (consistent with serenity's rustls)
• Extracted shared parse_config(raw, source) from load_config to avoid duplication
• Added load_config_from_url(url) async function with 10s timeout and clear error messages for network failure / non-2xx / invalid TOML
• main.rs checks if the first positional arg starts with http:// or https:// to decide the code path
• Env-var expansion (${VAR}) works identically on both local and remote config content

Test plan

• cargo build passes
• Existing local config path behavior unchanged
• agent-broker https://raw.githubusercontent.com/.../config.toml fetches and parses correctly
• Invalid URL returns clear error message
• Non-200 HTTP response returns clear error message
• Env-var expansion works on remote config content

[thepagent]

LGTM! 🙌 Clean and minimal implementation.

What I like:

• Auto-detect URL approach (no new flag needed) — exactly the kubectl-like UX we discussed in #80
• parse_config(raw, source) extraction is a nice refactor, avoids duplication between local and remote paths
• reqwest with rustls-tls — consistent with serenity's TLS backend
• 10s timeout + separate error messages for network failure / non-2xx / invalid TOML ✅
• Env-var expansion works on remote content — verified in tests
• main.rs change is only 8 lines — surgical

One minor nit (non-blocking):

• std::env::set_var / remove_var in tests can race in multi-threaded test runs. Not a real issue at this scale, but if you ever want to harden it, temp_env crate or serial_test can help. Totally fine to leave as-is for now.

Ship it! 🚀

needs further review

[masami-agent]

Hi @neilkuan, this PR overlaps with #160 which also implements remote config loading. Would you be okay closing one of these so we don't have two PRs for the same feature? Happy to discuss which one to keep. Thanks 🙏