Store agentWallet at sweep time and make it searchable

[NemboKid]

Pasting an agent's payment wallet into search finds nothing, because no table stores agentWallet. Rung 1 evidence carries ownerOf, not getAgentWallet, and the value cannot be reconstructed from events (see analysis/payments-per-chain.md). PR #17's q_match_sql doc comment states the precondition plainly: "Matching it means a sweep-time getAgentWallet read into a real column first, not a scan of evidence JSONB here." This issue is that sweep-time read and column.

Migration 0013_minter_capture.sql is the model to copy: additive and nullable, where NULL means "this run predates capture" and never "the agent has no wallet". Note the spec's semantics (spec/ERC8004SPEC.md, getAgentWallet): the value defaults to the owner and is cleared to the zero address on transfer, so a read zero address is a real observation, distinct from a failed read (NULL).

Where to look

• migrations/0013_minter_capture.sql — the pattern for an additive nullable snapshot column plus its partial (chain, …) index
• crates/chain/src/registry.rs — no getAgentWallet binding exists yet; add the contract call, read at the run's pinned block
• crates/sweeper/src/main.rs and crates/sweeper/src/store.rs — where snapshots are populated and persisted
• crates/api/src/routes/agents.rs — q_match_sql, the one definition of what q matches. PR API: cross-run search — q across explicit runs, grouped by chain #17 (branch journeys/cross-chain-search) moves this into a shared fragment used by both /api/agents and /api/search; the doc comment there explains why agentWallet is deliberately not matched today. Coordinate with API: cross-run search — q across explicit runs, grouped by chain #17 — the shared-fragment code refs land when it merges.
• crates/checks/src/version.rs — the schema-version doc; a new snapshot column gets an entry, as schema 6 did

Done when

• A migration adds nullable agent_snapshots.agent_wallet with a partial index on (chain, agent_wallet) WHERE agent_wallet IS NOT NULL (mirror idx_agent_snapshots_minter)
• The sweeper populates it via getAgentWallet at the run's pinned block; a failed read leaves NULL, never a fabricated value; a read zero address is stored as read
• q_match_sql gains a wallet-prefix arm (same shape as the owner-prefix match), so both endpoints that share the fragment match it identically
• Pasting a known agent's wallet address into search returns that agent
• crates/checks/src/version.rs documents the schema bump