feat(nip34,nip22): add git-over-nostr collaboration (NIP-34) and NIP-22 comments

[naliyi]

Summary

• Adds nip34: NIP-34 (git stuff) end to end -- repository announcements (kind:30617) and state (kind:30618, both addressable), patches (kind:1617), pull requests (kind:1618) and PR updates (kind:1619), issues (kind:1621), status events (kind:1630-1633) with ResolveStatus/ResolveRevisionStatus helpers implementing the spec's status-resolution rules, user grasp lists (kind:10317), and nostr:// clone URL parsing/building.
• Adds nip22: NIP-22 (Comment) -- the generic kind:1111 threading note NIP-34 replies build on (scoped to a root event, addressable event, or NIP-73 external identifier). Broken out as its own package since it's not git-specific.
• Both packages follow this SDK's existing pattern: a dependency-free protocol library (New*/Parse*/Validate* per event kind, sentinel Err* vars) plus a relayreg subpackage that registers relay-side support and event validators. No nip34/client subpackage -- unlike Blossom/ecash, NIP-34 has no second transport to dial; publishing/subscribing goes through the existing generic nip01/relay-client plumbing.
• Adds utils.FormatATag, the build-side counterpart to the existing ParseATag, used by both new packages to construct "a" tag addresses.
• README: new "Implemented NIPs" entries, a usage example, and an updated relay-side-concerns package list. CHANGELOG: Unreleased/Added entries.

Test plan

• just check (build + vet + test) passes across the whole module
• 41 new unit tests across nip34/nip22 covering New/Parse/Validate round-trips and error paths (wrong kind, malformed addresses/pubkeys/pointers, missing required tags)
• README's new code example compile-checked standalone against this branch

[naliyi]

Expanded the README's NIP-34 section from one example to one subsection per operation (announce+state, patch series/revision/PR/PR-update, issue+threaded replies, all four status kinds + ResolveStatus/ResolveRevisionStatus, grasp list, all three nostr:// clone URL forms, and subscribing to a repo's activity from a relay).

Every example was compile-checked and run standalone against this branch -- which caught a real bug the original example had: otherPrivateKeyHex was 63 hex chars (odd length). go build/go vet don't catch that since hex decoding only happens at runtime inside Sign(); it would have panicked for anyone who copy-pasted it. Fixed, and now every literal key/ID used during verification was generated and length-checked programmatically rather than hand-typed, to rule out the same class of mistake elsewhere.