feat(peak_usage): add peak-time indicator module

[timoklein]

Summary

Closes #146.

• Adds cship.peak_usage module that shows ⏰ Peak when Anthropic's peak-time rate limiting is likely active (Mon–Fri 07:00–17:00 US Pacific by default)
• Zero new dependencies: uses Howard Hinnant's civil date algorithm and Tomohiko Sakamoto's day-of-week algorithm for DST-aware UTC→Pacific conversion
• Configurable start_hour/end_hour with validation warnings for invalid ranges
• 23 unit tests covering peak/off-peak, DST transitions, custom hours, weekday boundaries, and config validation

Configuration

[cship.peak_usage]
symbol = "⏰ "
style  = "fg:#e0af68"
# start_hour = 7   # optional, default
# end_hour = 17    # optional, default (use 24 for "through end of day")

Test plan

• cargo test --lib modules::peak_usage — 23 tests pass
• cargo test — full suite (343 unit + 66 integration) passes
• cargo clippy -- -D warnings — clean
• cargo build --release — clean
• Manual test: verified indicator appears in live statusline with start_hour=0, end_hour=24

🤖 Generated with Claude Code

[timoklein]

@stephenleo I've let Claude add it to the cost guardian example config, where I think it is a good fit. I wasn't able to generate a good example screenshot because I couldn't replicate the theme. We can also just remove it again, then the screenshot doesn't need to get updated.

Indicator shows up properly when you set start_hour=0 and end_hour=24 (debug setting).

One minor thing: CONTRIBUTING.md doesn't say which BMAD module you need to use /bmad-quick-dev. I just installed all.