Add dolt_config() for committer name and email

[timsehn]

Summary

Closes #147. Adds dolt_config() SQL function for setting committer name and email.

SELECT dolt_config('user.name', 'Tim Sehn');
SELECT dolt_config('user.email', 'tim@dolthub.com');
SELECT dolt_commit('-m', 'my commit');
-- committer: Tim Sehn, email: tim@dolthub.com

• Per-session config (stored on Btree struct, same pattern as zBranch)
• All 4 commit-creating operations now use config: dolt_commit, dolt_merge, dolt_cherry_pick, dolt_revert
• --author flag on dolt_commit still works as per-commit override
• dolt_config('key') returns current value
• Default: "doltlite" with empty email (unchanged behavior if config not set)

Test plan

• Default author shows "doltlite"
• Config set changes commit author
• --author flag overrides config
• Merge commits use config
• Config is per-session (not persisted across connections)

🤖 Generated with Claude Code

[github-actions]

Sysbench-Style Benchmark: Doltlite vs SQLite

In-Memory

Reads

Test	SQLite (ms)	Doltlite (ms)	Multiplier
oltp_point_select	49	76	1.55
oltp_range_select	31	36	1.16
oltp_sum_range	13	15	1.15
oltp_order_range	5	6	1.20
oltp_distinct_range	7	7	1.00
oltp_index_scan	6	55	9.17
select_random_points	22	69	3.14
select_random_ranges	7	9	1.29
covering_index_scan	12	68	5.67
groupby_scan	53	58	1.09
index_join	6	21	3.50
index_join_scan	3	255	85.00
types_table_scan	11	12	1.09
table_scan	1	2	2.00
oltp_read_only	207	236	1.14

Writes

Test	SQLite (ms)	Doltlite (ms)	Multiplier
oltp_bulk_insert	27	40	1.48
oltp_insert	21	31	1.48
oltp_update_index	48	614	12.79
oltp_update_non_index	33	46	1.39
oltp_delete_insert	47	333	7.09
oltp_write_only	18	265	14.72
types_delete_insert	24	33	1.38
oltp_read_write	123	433	3.52

File-Backed

Reads

Test	SQLite (ms)	Doltlite (ms)	Multiplier
oltp_point_select	134	89	0.66
oltp_range_select	59	40	0.68
oltp_sum_range	22	19	0.86
oltp_order_range	10	6	0.60
oltp_distinct_range	7	7	1.00
oltp_index_scan	15	57	3.80
select_random_points	34	74	2.18
select_random_ranges	17	13	0.76
covering_index_scan	22	66	3.00
groupby_scan	53	58	1.09
index_join	14	21	1.50
index_join_scan	3	257	85.67
types_table_scan	13	13	1.00
table_scan	2	3	1.50
oltp_read_only	315	283	0.90

Writes

Test	SQLite (ms)	Doltlite (ms)	Multiplier
oltp_bulk_insert	30	39	1.30
oltp_insert	22	34	1.55
oltp_update_index	47	678	14.43
oltp_update_non_index	37	49	1.32
oltp_delete_insert	45	327	7.27
oltp_write_only	23	269	11.70
types_delete_insert	26	31	1.19
oltp_read_write	139	415	2.99

10000 rows, single CLI invocation per test, workload-only timing via SQL timestamps.