#1129 Docs: document user-global telemetry (telemetry.yaml opt-out, spool location, telemetry stats behavior)

Follow-up to #1125 (telemetry is now user-global). The manual under content/manual/reference/ doesn't yet cover the new behavior:

• Spool location: telemetry now lives at $XDG_CONFIG_HOME/swamp/telemetry (default ~/.config/swamp/telemetry), not repo-local .swamp/telemetry. 'swamp repo init' no longer creates a repo-local telemetry directory.
• New persistent opt-out: ~/.config/swamp/telemetry.yaml with 'disabled: true' suppresses telemetry for repo-less runs (durable equivalent of a repo marker's telemetryDisabled). SWAMP_NO_TELEMETRY and --no-telemetry still apply everywhere. repository-configuration.md documents the existing opt-outs and should mention this one.
• 'swamp telemetry stats' now reads the user-global spool, runs outside a repo, and no longer accepts --repo-dir. Any doc/example showing 'telemetry stats --repo-dir' is stale.
• 'swamp repo upgrade' migrates unflushed entries from a legacy repo-local spool to the global spool (skipping repos that opted out); the old repo-local folder is left orphaned.

Suggested home: content/manual/reference/repository-configuration.md plus wherever telemetry stats is referenced.