fix: make client-owned web fetch discoverable (#252)

[ankitranjan7]

Fixes #252.

webcmd web fetch always worked — it is intercepted on the fast path in src/main.ts:79 before adapter discovery — but nothing in the CLI's own surface said it existed unless you had installed the web plugin, and webcmd web fetch -h threw an ArgumentError about --url.

What changed

• clis/web/fetch.js registers the command through a new makeWebFetchCommand() factory, so build-manifest and the runtime filesystem scan both pick it up. src/discovery.ts's module pattern now matches make<Pascal>Command(, the same convention src/build-manifest.ts:48 already used.
• cli-manifest.json gains the generated web/fetch entry (regenerates byte-identical; check:hosted-contract passes, hosted-contract.json unchanged since the command is not browser-based).
• The fast path stays exactly where it was — execution never touches the registry, so hosted mode still never cloud-routes this command.
• Now that help is real, the fast path honours what it advertises: -f/--format for output (markdown remains the default), structured --help -f yaml|json, an error for an unsupported format instead of a silent table fallback, and an error for a flag-shaped --timeout/--max-chars value instead of coercing it to 1.

Verified against a rebuilt dist

Surface	Before	After
webcmd --help	no web	web under Site adapters
webcmd list	absent	fetch [public] … [builtin]
webcmd list -f json	absent	full entry with all four args
webcmd web fetch -h	ArgumentError	real help
webcmd --get-completions web ''	empty	fetch, fetch-browser

Four new tests in src/fetch/command.test.ts cover the -f output path, structured help, the format rejection, and the --timeout -5 rejection. Full suite passes except tests/e2e/plugin-management.test.ts, which clones plugins over the network and fails identically on main.

Out of scope

• [Bug]: webcmd web fetch prints a raw Node stack trace instead of a formatted CLI error #246 — ArgumentError still prints a raw stack trace on the fast path.
• [Bug]: web fetch-browser is not installed by default, so the FETCH_BLOCKED escalation path is a dead end #247 / clis/ not being in package.json files. Unreachable while the argv[0] === 'web' && argv[1] === 'fetch' prefix holds, and pre-existing for fetch-browser.

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