Fix burn overhead deltas filtering and unsupported flags

[willwashburn]

Problem

burn overhead deltas is exposed under burn overhead, so it inherits --project, --kind, and the shared --since help text. In the current implementation, project and kind are dropped when dispatching to deltas, and --since only accepts relative ranges. ISO timestamps or invalid values silently become None.

A session-specific deltas query also ignores since at the SDK layer because ContextDeltaOpts::session bypasses the seed query. The SDK currently uses since only to select sessions, not to filter returned deltas within a session.

Relevant files

• crates/relayburn-cli/src/commands/overhead.rs
• crates/relayburn-cli/src/cli.rs
• crates/relayburn-sdk/src/query_verbs.rs
• crates/relayburn-sdk/src/analyze/context_delta.rs

Suggested scope

Keep this small and choose one clear contract:

• Either make overhead deltas support the shared --since contract, including ISO timestamps, and reject invalid input.
• Or remove/override inherited flags and help text so only supported options are advertised.

Also decide whether --project and --kind should be real filters for deltas. If not, they should not be accepted on this subcommand.

Definition of done

• burn overhead deltas --help only advertises supported flags.
• Invalid --since values do not silently widen the query.
• --since behavior is identical for all-session and --session <id> modes, or the difference is explicitly rejected and documented.
• Add CLI and SDK regression tests around relative range, ISO timestamp or rejection, and session-specific filtering.