burn compare: add --provider filter (finish #83)

[willwashburn]

Context

PR #124 (closed #31) added the synthetic-provider CLI surface for `summary` / `by-tool` / `waste` — `--provider ` filter on all three, and `--by-provider` grouping on `summary`. `burn compare` was deferred:

`compare --by-provider` is explicitly deferrable per #83.

But `compare --provider ` (the higher-leverage filter half) was not added, and #83's acceptance asks for it across all four commands.

The helper functions already exist in `packages/cli/src/provider.ts` (`parseProviderFilter`, `filterTurnsByProvider`, `resolveTurnProvider`) — the rewire surface is small.

Proposal

In `packages/cli/src/commands/compare.ts`:

1. Accept `--provider ` (single value or comma-separated list, matching summary's syntax).
2. Run `filterTurnsByProvider(turns, parseProviderFilter(args.flags.provider))` against the input slice before passing to `buildCompareTable`.
3. When the filter excludes all input, render the same empty-table message the existing `--models` filter produces.
4. Cover via the existing parity-test pattern in `packages/cli/src/commands/compare.test.ts` (or `packages/analyze/src/compare-archive.test.ts` if the filter is best applied at the SQL layer for the archive path).

Optionally — keep simple, defer if it complicates:

5. Extract `aggregateByProvider` from the inline call site in `summary.ts` into `@relayburn/analyze` so multiple commands can share it. Issue Wire synthetic reattribution into rendering: --provider filter and --by-provider grouping across summary/compare/by-tool/waste #83 explicitly asks for this.
6. `compare --by-provider` grouping. Wire synthetic reattribution into rendering: --provider filter and --by-provider grouping across summary/compare/by-tool/waste #83 marks this as a stretch goal.

Acceptance criteria

• `burn compare --provider synthetic` filters the compare table to only synthetic-routed turns.
• Multi-value (`--provider synthetic,openrouter`) works, matching summary's parser.
• Empty-result rendering matches the existing `--models` empty path.
• Tests cover the filter on both the in-memory and (if PR Migrate burn compare to grouped SQL aggregates (#88) #130 has merged) the archive code paths.

Out of scope

• `compare --by-provider` (acceptable to defer per Wire synthetic reattribution into rendering: --provider filter and --by-provider grouping across summary/compare/by-tool/waste #83).
• Adding new providers to the reattribution table (covered by Extend synthetic reattribution rules to OpenRouter and other aggregator prefixes #90 OpenRouter).

Refs

• Spun out of Wire synthetic reattribution into rendering: --provider filter and --by-provider grouping across summary/compare/by-tool/waste #83 (PR Add Synthetic provider CLI grouping #124 partially landed it)
• Helper module: `packages/cli/src/provider.ts`
• Sibling commands already with the filter: `summary.ts`, `by-tool.ts`, `waste.ts`