Add age-tiered channel memory digests

[mostlydev]

Summary

• add budget.raw_recent to channel-memory /digest so deterministic raw_excerpt blocks only serve inside a recent tier
• preserve hard events, tombstones, sparse deterministic rollups, and LLM summary blocks across the requested digest horizon
• have claw-wall request a six-hour raw tier and surface digest_raw_recent plus digest_older_raw_omitted in the channel-awareness header
• query newest digest candidates first so older raw history cannot starve recent useful blocks

Tests

• go test ./examples/channel-memory
• go test ./cmd/claw-wall
• go test ./...

Stacked on #328 for #260.

Closes #260