Fix default hourly cost quota to unlimited instead of $1.00

[cryptonahue]

Summary

• ResourceQuota::default() had max_cost_per_hour_usd hardcoded to 1.0, causing agents to hit an unexpected $1/hour hard stop even when no budget was explicitly configured by the user
• max_cost_per_day_usd and max_cost_per_month_usd already defaulted to 0.0 (unlimited) — this PR makes max_cost_per_hour_usd consistent with them
• Changed default from 1.0 to 0.0 (unlimited)

Reproduction

1. Install OpenFang via the install script on a fresh server
2. Create any agent (or use the default dona template)
3. Have an active conversation that exceeds $1.00 in LLM costs within an hour
4. Agent stops with: Agent error: Resource quota exceeded: ... exceeded hourly cost quota: $1.XXXX / $1.0000
5. The Budget settings page shows all agents capped at $1.0000 hourly even with global limits set to unlimited

Fix

// Before
max_cost_per_hour_usd: 1.0,

// After  
max_cost_per_hour_usd: 0.0, // unlimited by default

Test plan

• cargo test -p openfang-types — 287 tests pass
• cargo clippy -p openfang-types -- -D warnings — zero warnings
• cargo fmt --all --check — no diff on changed file