Release 1.5.1: drop the last dependency, add a help system, correct the docs

[umudhasanli]

Zero dependencies, for real this time

package.json declared undici-types as a runtime dependency, so
npm install flare-redact pulled a package down with every copy — while the
README carried a "dependencies 0" badge and the description said
"Zero-dependency". Nothing in src/ ever imported it.

It is gone. A clean install now brings exactly one package, and a TypeScript
consumer still compiles: the type it existed to satisfy comes from the
consumer's own @types/node, which is where it belonged.

ts-consumer@1.0.0
├─┬ @types/node@20.19.43
│ └── undici-types@6.21.0      <- theirs, not ours
├── flare-redact@1.5.1
└── typescript@5.9.3

The npm package also carries no Python, Go or Rust code — the files list never
included sdk/, and this PR says so explicitly in the README so nobody has to
check.

A CLI that answers questions

flare-redact help [topic]. A flag list tells you a flag exists and nothing
about when to reach for it, so there are ten topics that explain the thing
behind the flag: detectors, modes, vault, scan, files, gateway,
library, sdks, env, exit-codes.

$ flare-redact help modes
MODES — what replaces a matched value

  --mode mask        the default. Partial, shape-preserving: a***@***, sk-***
  --mode hash        keyed HMAC-SHA-256. Same input, same output, so you can
                     still count and join on it. Requires a key.
  …

Any detector id is a topic too — flare-redact help email gives what it finds,
whether it runs by default, its tags, and how to select or drop it. A name that
matches nothing suggests the nearest one rather than failing with nothing to go
on, goes to stderr, and exits 2 so it never lands in a piped report.

flare-gateway help and flare-redact gateway help now work alongside
--help, and an unknown flag on either CLI points at the help instead of only
complaining.

Documentation that was wrong

• The Python and Rust installs did not work. pip install flare-redact and
  flare-redact = "1.5" were documented in the README, both SDK READMEs and the
  release notes, but neither package is on PyPI or crates.io — both return 404.
  Replaced with the repository installs, which are now tested end to end:
  pip install from the git subdirectory imports and redacts, and a Cargo git
  dependency builds and runs. The Go module was and remains installable with
  go get.
• The README badge claimed 56 detectors; there are 82.
• The CI example and the docs site still pinned 1.4.1, and the playground
  bundle on the docs site was still the 1.4.x build. All three now track the
  release, and the docs-site test that pinned the old label was updated with
  them.
• bin/flare-gateway.mjs was committed without the executable bit that
  bin/flare-redact.mjs has.

Verification

Node	354 tests (13 new, covering the help system)
Python	87 · Go 18 · Rust 19
Lint	go vet, clippy --all-targets -D warnings
Spec	drift check and conformance build
Packaging	npm ci, npm audit, npm pack, and a TypeScript consumer compiling against a clean install of the tarball
Examples	all three smokes