feat(usage_limits): per-window session/weekly tokens and a {bar} placeholder

[nh13]

Adds two capabilities to [cship.usage_limits], both opt-in and backward-compatible.

Per-window tokens

$cship.usage_limits.session and $cship.usage_limits.weekly (aliases .five_hour / .seven_day) render a single standard window, colored by that window's own utilization — unlike $cship.usage_limits, whose threshold color keys off max(5h, 7d). This lets the session and weekly limits sit on separate lines and escalate (green → yellow → red) independently.

{bar} placeholder

A proportional Unicode bar usable in five_hour_format / seven_day_format, with bar_width, bar_filled_char, and bar_empty_char config (mirrors context_bar's fill math). It is scoped to the per-window formats only — the per-model and extra-usage formats leave {bar} literal. Combined with the per-window tokens, each window can show its own progress bar:

[cship]
lines = ["$cship.usage_limits.session", "$cship.usage_limits.weekly"]

[cship.usage_limits]
five_hour_format   = "5h {bar} {pct}%"
seven_day_format   = "7d {bar} {pct}%"
bar_width          = 20
warn_threshold     = 50
warn_style         = "yellow"
critical_threshold = 80
critical_style     = "bold red"

Notes

• Default $cship.usage_limits output is unchanged — the shared per-window formatter produces byte-identical text when no {bar} is present, and show_per_model / extra-usage / separator handling is preserved.
• bar_width is clamped to a sane maximum, and the bar string is built only when {bar} is actually referenced.
• render_bar is kept local with a "keep in sync" note rather than refactoring context_bar in this feature PR — happy to extract a shared helper if you'd prefer.
• cship explain reports the per-window tokens as plan-unavailable on Enterprise (alongside the per-model sub-tokens) instead of a misleading fetch-failure hint.

Docs & tests

docs/configuration.md, docs/faq.md, README.md, and the committed config-schema.json are updated. New unit tests cover bar rendering (incl. 0-width, clamping, floor semantics), placeholder substitution, the {bar} negative scope in per-model/extra formats, independent per-window coloring, and the aliases. cargo fmt, clippy --all-targets (0 warnings), and the full suite pass.