feat(lark): expose task sender identity to agents

[aihes]

Summary

• attest the direct human channel sender while building a daemon task claim
• inject generic channel identity metadata into the task process environment, plus MULTICA_LARK_OPEN_ID for Feishu
• keep credential routing task-token based and omit identity on missing or ambiguous bindings

Safety boundaries

The server revalidates the chat binding, direct-human attribution, active installation, workspace and agent ownership, current membership, and exactly one installation-scoped user binding. The daemon accepts only a complete, channel-consistent tuple. Custom environment settings cannot override MULTICA_* values. Logs contain the task ID, channel type, and resolution reason, never the provider user ID.

No table, migration, external API, or execenv preparation payload changes are included. Old servers and daemons remain compatible through the optional claim field.

Validation

• make sqlc
• go test -race ./internal/daemon ./internal/handler
• go test ./pkg/agent/...
• go vet ./...
• git diff --check
• full bash scripts/test-go.sh --race: all reached packages passed except the existing PostgreSQL-specific internal/metrics/TestBusinessSamplerStatementTimeoutCutsHungQuery, where local PostgreSQL returned a client context deadline exceeded rather than the expected server PgError; focused reruns reproduce it without this change

Rollout

Mac mini rollout will rebuild backend and daemon from one integration commit and update the existing task-routed Lark Skill. The new environment metadata remains informational; Lark credentials continue to be selected only by multica-lark-task-router from MULTICA_TOKEN.

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