feat: anonymous adoption telemetry on first client.New

Sends one fire-and-forget POST per process the first time a consumer
constructs a Bot via client.New. Helps track real-world adoption and
version spread of the library. No identifiers, no API contents.

Implementation:
- new client/version.go: exported Version var (currently 0.7.11)
- new client/telemetry.go: sync.Once gate + telemetry.Send call
- client.go New(): single fireTelemetryOnce() line at function entry
- client/telemetry_test.go: TestMain disables outgoing pings during the
  library's own test suite

README §Telemetry documents the payload, the opt-out env vars, and links
to the upstream oss-telemetry source so consumers can audit what is sent.

Opt out via any of:
  DO_NOT_TRACK=1
  OSS_TELEMETRY_DISABLED=1
  GO_TELEGRAM_DISABLE_TELEMETRY=1
  osstelemetry.Disable() before the first client.New

go.sum

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 github.com/klauspost/compress v1.18.6/go.mod h1:cwPg85FWrGar70rWktvGQj8/hthj3wpl0PGDogxkrSQ=
+github.com/lukaszraczylo/oss-telemetry v0.0.0-20260521005811-e02d51419c52 h1:HAm1OV/1uYN3VA/HdDNFjwh8KerTLwl1SoxF+IiNf/M=
+github.com/lukaszraczylo/oss-telemetry v0.0.0-20260521005811-e02d51419c52/go.mod h1:+Cn78qZo8rc3T9eZt0v3oICYRdd75wORtSidc8lNjDQ=
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@@ -16,6 +18,8 @@ github.com/valyala/bytebufferpool v1.0.0 h1:GqA5TC/0021Y/b9FG4Oi9Mr3q7XYx6Kllzaw
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+github.com/xyproto/randomstring v1.0.5 h1:YtlWPoRdgMu3NZtP45drfy1GKoojuR7hmRcnhZqKjWU=
+github.com/xyproto/randomstring v1.0.5/go.mod h1:rgmS5DeNXLivK7YprL0pY+lTuhNQW3iGxZ18UQApw/E=
 golang.org/x/net v0.54.0 h1:2zJIZAxAHV/OHCDTCOHAYehQzLfSXuf/5SoL/Dv6w/w=
 golang.org/x/net v0.54.0/go.mod h1:Sj4oj8jK6XmHpBZU/zWHw3BV3abl4Kvi+Ut7cQcY+cQ=
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