feat(acp): inject requester git email into [Context] for commit trailers

[wpfleger96]

Summary

Agents currently fall back to git config user.email for commit trailers, which resolves to the repo owner's identity — not the human who triggered the turn. NIP-01 kind:0 already has a standard email field; this change stores it and surfaces it in every prompt so agents can use the correct identity.

Changes

DB migration

• migrations/0004_user_git_email.sql: adds nullable git_email TEXT column to users table

buzz-db

• UserProfile gains a git_email: Option<String> field
• update_user_profile accepts an additional git_email: Option<&str> parameter; the dynamic SQL builder handles it the same way as other nullable profile fields (empty string → NULL)
• get_user and get_user_by_nip05 SELECT the new column
• Db::update_user_profile wrapper updated to match

buzz-relay

• handle_kind0_profile parses the NIP-01 standard email field from kind:0 content and passes it to update_user_profile; no domain validation (unlike NIP-05) — the user is responsible for setting a valid git email
• The NIP-05 collision retry path also forwards git_email so it is written even when the NIP-05 handle is contested

buzz-acp

• PromptProfile gains git_email: Option<String>
• parse_kind0_profile_lookup extracts the email field (empty strings filtered to None)
• format_context_hints accepts sender_pubkey and profile_lookup; emits Requester-Git-Email: Name <email> when the triggering event's author has a git_email set, omits the line entirely when absent

nest_agents.md

• "Discovering the human's identity" rule updated: prefer Requester-Git-Email: from [Context] when present; fall back to git config user.email when absent

Tests

• test_parse_kind0_profile_lookup_extracts_email — email field parsed into git_email
• test_parse_kind0_profile_lookup_ignores_empty_email — empty string treated as absent
• test_format_context_hints_emits_requester_git_email_when_present — line appears in [Context]
• test_format_context_hints_omits_requester_git_email_when_absent — line absent when no email
• test_format_context_hints_requester_git_email_uses_nip05_as_name_fallback — name falls back to nip05_handle
• embedded_migrator_contains_all_schema_migrations updated for migration 4

Notes

Profile UI for setting email is out of scope for this PR. Users can set it in any Nostr client via kind:0; the relay syncs it on receipt. A profile settings field can be a follow-up.