feat(cost): auto-format duration fields as human-readable

[stephenleo]

Summary

• Render $cship.cost.total_duration_ms and $cship.cost.total_api_duration_ms as human-readable durations (45s, 1m30s, 2h15m, or 750ms for sub-second) instead of raw millisecond integers. Mirrors starship's opinionated cmd_duration output — no new config flags.
• Add $cship.cost.total_duration and $cship.cost.total_api_duration as accepted aliases for the _ms-suffixed names, on both surfaces: format-string variables and TOML config keys.
• Threshold comparisons still operate on raw milliseconds, so existing warn_threshold = 30000.0 configs keep working unchanged.

Closes #162.

Breaking change

Users with a custom format = "[$value ms]($style)" on either of the duration sub-fields will now see "45s ms". Migration is a one-character edit: drop the literal ms from the format string. CHANGELOG entry added under [Unreleased] calling this out explicitly.

Design notes

• Followed starship's fully-opinionated path (single human-readable representation, no raw-ms escape hatch). cship is starship-flavored by design and the goal was to keep the config surface small.
• format_duration_ms uses integer u64 arithmetic throughout — no f64, no decimal drift, truncation is intentional and matches starship's whole-second granularity.
• Sub-second values (< 1000ms) keep ms precision so a fast 500ms response doesn't read as "0s".
• Aliases route to a single render function via a | pattern in the dispatch match — no duplicated logic.
• Config aliases use #[serde(alias = "...")] on the existing field — canonical Rust name stays total_duration_ms, so no cascading code changes.

Test plan

• cargo fmt --check — clean
• cargo clippy --all-targets -- -D warnings — clean
• cargo test — 387 lib + 68 integration tests pass (added 11 new formatter tests, 2 dispatch-alias tests, 2 config-alias tests; updated 5 cost tests + 1 integration test for new format)
• cargo build --release — clean
• Manual smoke (default path): total_duration_ms = 45000 → 45s
• Manual smoke (sub-second): total_duration_ms = 750 → 750ms (not 0s)
• Manual smoke (threshold still works): warn_threshold = 30000.0 fires yellow ANSI on 45s
• Manual smoke (no regression on shared codepath): $cship.cost.total_lines_added still renders as 156
• Manual smoke (variable alias parity): $cship.cost.total_duration_ms and $cship.cost.total_duration produce byte-identical output
• Manual smoke (config-key alias parity): [cship.cost.total_duration_ms] and [cship.cost.total_duration] produce byte-identical warn-styled output

🤖 Generated with Claude Code

[stephenleo]

Wait for #153 as the intent might be duplicated