Limit proxies list to 100 proxies by default

[meznaric]

The problem

webshare proxies list --mode=backbone looks like it hangs. It doesn't — it is paginating the entire proxy list.

Three things compound:

1. --limit defaulted to 0 = "all", so the loop drove Proxies.ListAll through every page via the envelope's next URL, sequentially.
2. Results were buffered into a slice and only written after the loop finished — no partial output, no progress, nothing on stderr.
3. A residential backbone plan holds millions of entries at the server's default page size, so that is thousands of round trips before the first byte of output.

Backbone makes it worse: only mode and country_code__in work as filters there (spec/notes/proxy.md), so most ways a user would try to narrow the list are silently ignored while still paying for the full walk.

The change

• --limit now defaults to 100, matching the existing default on proxies replaced. --limit 0 restores the old fetch-everything behavior.
• When the limit cuts the list short, the command says so on stderr — so the cap is never silent, and the notice never corrupts a piped address:port:username:password stream.
• Help text and README point at proxies download for full exports: the server renders the whole list in a single request, which is the right tool for that job.
• README examples that redirect to a file now pass --limit 0, so they keep producing complete lists.

Behavior

Against a fake API serving a residential-sized list (6M entries, 60/page):

	before	after
proxies list --mode=backbone	no output after 60s+	100 rows in 8ms
proxies list --mode=backbone --limit 0	same as before	same as before

Tests

Three new tests drive a multi-page fake API and assert on the number of pages actually fetched, not just the output:

• default limit stops after 2 pages of 60 rather than walking all 100
• --limit 0 still fetches every page
• a list shorter than the limit is returned whole, with no truncation notice

Verified the cap is what's doing the work: with defaultProxyListLimit set back to 0, TestProxiesListStopsAtDefaultLimit never completes.

Breaking change

Scripts relying on proxies list returning the complete list need --limit 0 (or, better, proxies download). This is worth a minor version bump — proposing v0.2.0.

[meznaric]

Heads-up: this PR also unblocks CI

The first run here failed lint for a reason unrelated to the change:

internal error in importing "math/bits" (cannot decode "math/bits",
export data version 4 is greater than maximum supported version 2)

CI pins staticcheck@2026.1 (v0.7.0) but installs go-version: stable, which is now Go 1.27.1. staticcheck v0.7.0 cannot read Go 1.27's export data, so it dies on stdlib packages before reaching any repo code.

Verified pre-existing: running that exact staticcheck version against an unmodified origin/main in a clean worktree reproduces the same five errors. main has been red since Go stable moved past 1.26 — the last green run on it was 2026-07-28.

Bumped the pin to 2026.2.1, which supports the current toolchain and reports nothing on this repo (checked on both main and this branch).

Worth considering separately: the pinned linter against a floating stable Go will rot again the next time Go bumps its export format.