Add mapbox isochrone

[mattpodwysocki]

⚠️ Temporarily based on #43, not main

This reuses the flattening mechanism (spec::FLATTENED_SERVICES),
ARG_NAME_OVERRIDES, and the path_segment_for/UNESCAPED_PATH_PARAMS
fix #43 introduces, since both APIs' profile path parameter hits the
same global-flag collision and the same free-form-profile requirement.
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, the second Navigation-category API with no prior CLI
coverage. Same shape as #43: hand-authored into custom-openapi/ since
openapi-specs publishes no spec for this API either.

No subcommand: like mapbox directions, this API has one operation, so
there's nothing a second word (the old contours) would disambiguate.
Same shape mapbox usage already has.

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.

Routing profile is free-form here too

Same fix as #43: an earlier version of this command validated profile
against the four documented values client-side, via a clap enum. Some OEM
accounts have additional profiles that aren't published, so that
validation would have broken this command for exactly the accounts that
most need it. profile is now sent exactly as typed, and reaches the URL
unescaped through UNESCAPED_PATH_PARAMS (("isochrone", "profile"))
rather than relying on the enum-implies-safe assumption that stopped
holding once the enum came off.

Also included

A fix for a --help regression 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, plus the bare mapbox isochrone command shape
itself (no subcommand accepted, --schema reflects the new name).

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

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