feat(usage_limits): add {reset_at} absolute reset-time placeholder

[Romboter]

Adds a {reset_at} placeholder to $cship.usage_limits (five-hour, seven-day, and per-model formats), giving an absolute local clock time alongside the existing relative {reset} countdown.

• Format — clock-only if the reset falls on today's local date (e.g. 7:42 PM), weekday-prefixed otherwise (e.g. Mon 9:00 AM); ? when the reset time is unknown, now when it's already past. Mirrors {reset}'s None/past handling.
• New dependency — chrono (clock feature only). Std has no portable local-timezone/DST API, and hand-rolling that conversion is a correctness trap; chrono::Local is the standard, minimal way to get it right.
• Wired into format_output (five-hour/seven-day sections) and format_single_model (per-model breakdown) alongside the existing {pct}/{remaining}/{reset}/{pace} substitutions.
• Docs updated: docs/configuration.md placeholder table + example block, and cship.toml (repo's own showcase config) updated to demonstrate {reset_at} in both formats.

Verification

• cargo fmt --check ✅
• cargo clippy -- -D warnings ✅
• cargo test ✅ (476 unit + 73 integration)
• cargo build --release ✅

🤖 Generated with Claude Code

[stephenleo]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

Confirmed the {reset_at} placeholder does not already exist on main — the existing {reset} renders a relative countdown (e.g. 4h12m), whereas {reset_at} is a new absolute local clock time (e.g. 7:42 PM / Mon 9:00 AM).

🤖 Generated with Claude Code

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[stephenleo]

Thank you for your submission!