fix(observability): cap chatty fold_db tracing channels to keep observability.jsonl tractable

[shiba4life]

Summary

• $FOLDDB_HOME/observability.jsonl grows ~25 MB / 10 min (~150 MB/h) on a fresh node doing routine ingestion, with no rotation. The upstream observability crate writes a plain append-only file with no rotation knob, so until that's added upstream, we cap the noisiest channels at the node side.
• Two fold_db tracing::* channels dominated ~72% of volume on a 2026-05-09 dogfood (132 Apple Notes + 5 photos + smart-folder of 3 files): fold_db::fold_db_core::mutation_manager (~60%) and fold_db::db_operations::atom_store (~12%). Both are noise unless you're explicitly debugging batch-mutation or atom storage.
• This PR extends src/log_filter.rs (which already caps sled at INFO ahead of upstream init) with a NATIVE_DEFAULT_CAPS table covering those two targets, opt-in via FOLDDB_LOG_MUTATION_MANAGER=<level> / FOLDDB_LOG_ATOM_STORE=<level>. augment_rust_log is generalized to accept any target=level directive (was sled-only).
• run.sh now honors an operator-supplied RUST_LOG (${RUST_LOG:-debug}) instead of unconditionally clobbering it. CLAUDE.md gets a short note under "Observability conventions" pointing at the new env vars.

Out of scope (per the task prompt): adding rotation to the upstream observability crate. If we want that, it should be a follow-up in ~/code/edgevector/fold_db adding tracing_appender::rolling to layers/fmt::build_fmt_writer, then a rev bump here.

Test plan

• cargo fmt --all -- --check clean
• cargo check --workspace clean (only the pre-existing macOS-only build_attendee_ingestion_records dead-code warning, gated #[cfg(target_os = "macos")] so CI on Linux doesn't see it)
• cargo nextest run --workspace --lib — 1006 tests pass; 13 log_filter::tests pass including two new ones (appends_native_tracing_directive_with_module_path, default_directives_include_all_caps_at_default_levels)
• cargo test --workspace --doc clean
• scripts/lint-no-hardcoded-urls.sh clean
• Manual smoke: run a fresh node for a few minutes; confirm mutation_manager / atom_store DEBUG lines no longer appear in observability.jsonl and that FOLDDB_LOG_MUTATION_MANAGER=debug restores them

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