feat: add retain_attestations API and CLI purge command for removing stale pending attestations

[Copilot]

Adds a generic retain_attestations primitive for filtering attestation leaves in timestamp trees, a purge CLI command with interactive selection, and syncs across all SDKs.

Core (crates/core)

• Timestamp::retain_attestations<F>(&mut self, f: F) — generic filter taking FnMut(&RawAttestation) -> bool, returns true to keep. Collapses single-branch FORKs after removal.
• Timestamp::retain_attestations_mut<F>(&mut self, f: F) — same as above but provides &mut RawAttestation access, allowing in-place modification of attestations during filtering.
• Timestamp::purge_pending() — convenience wrapper removing all pending attestations.

// Generic: remove any attestation type
ts.retain_attestations(|att| att.tag != PendingAttestation::TAG);

// Mutable variant: modify attestations during filtering
ts.retain_attestations_mut(|att| { /* can mutate att */ true });

// Convenience
ts.purge_pending();

Rust SDK (packages/sdk-rs)

• Sdk::list_pending(), Sdk::purge_pending(), Sdk::purge_pending_by_uris(stamp, uris) — built on retain_attestations

CLI (crates/cli)

• uts purge <FILES> — interactive numbered selection of which pending attestations to remove
• Accepts comma-separated indices, all, or none; -y/--yes skips prompt
• If purging would leave zero attestations, warns and does not touch the file

TypeScript SDK

• retainAttestations(stamp, shouldRetain) — generic
• purgePending(stamp, urlsToPurge?) — convenience with optional selective filter

Go SDK

• RetainAttestations(stamp, shouldRetain) — generic
• PurgePending(stamp) / PurgePendingByURIs(stamp, uris) — convenience

Python SDK

• retain_attestations(stamp, should_retain) — generic
• purge_pending(stamp, uris_to_purge=None) — convenience
• New PurgeResult dataclass

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