Add mapbox isochrone contours

[mattpodwysocki]

⚠️ Temporarily based on #43, not main

This reuses ARG_NAME_OVERRIDES and the path_segment_for fix #43
introduces — both APIs' profile path parameter hits the same global-flag
collision. Once #43 merges, this should be retargeted to main (gh pr edit --base main) rather than reviewed against it as a diff. Everything
below is scoped to what this PR actually adds on top of #43.

What

mapbox isochrone contours — the second Navigation-category API with no
prior CLI coverage (see the private tracker's api-coverage audit). Same
shape as #43: hand-authored into custom-openapi/ since openapi-specs
publishes no spec for this API either.

How far you can get from a point in a given time or distance, for driving
(with or without live traffic), walking, or cycling — as GeoJSON polygons
or linestrings.

contours_minutes and contours_meters are mutually exclusive but neither
is individually required by this CLI's own validation — the same "not
enforced before the request goes out" precedent search category already
uses for its own proximity/near/bbox/route disjunction. The API answers 422
if both or neither are given.

Also included

A fix for a --help regression I found while writing this, in code #43
already merged into this branch: first_sentence() in src/main.rs cuts a
--help line at the first ., and eight parameter descriptions in
directions.yaml led with a complete sentence before the substantive
content (`mapbox/driving` only. etc.), eating everything after it.
isochrone.yaml's own denoise had the same shape (0.0-1.0: ...) and
would have rendered as literally 0. Fixed all nine by moving the
qualifier to the end of the description instead of the front — --schema
and docs/commands.md were never affected, since they show descriptions in
full.

Verification

Smoke-tested against production: real contour polygons and linestrings for
mapbox/driving and mapbox/walking, --polygons, and
--contours-minutes with multiple values, all verified to return the
documented GeoJSON shape.

485 tests, cargo fmt --check and cargo clippy --all-targets -- -D warnings both clean.

🤖 Generated with Claude Code