fix: only run build/release jobs on tag push

[darach]

workflow_dispatch run was falling through to build/release where it failed because GITHUB_REF is main, not a tag. Build jobs now only run on tag push.

[github-actions]

✅ Benchmark Results: No significant regressions

Click to expand benchmark comparison

group                                         main                                    pr
-----                                         ----                                    --
diff_patch_merge/crdt_lww/batch/100           1.00    117.1±2.59ns        ? ?/sec     1.23    144.1±3.30ns        ? ?/sec
format/comparison/toml/simd                   1.22      3.4±0.04µs 288.6 KElem/sec    1.00      2.8±0.38µs 350.7 KElem/sec
format/comparison/yaml/simd                   1.27      4.7±0.06µs 209.1 KElem/sec    1.00      3.7±0.05µs 264.8 KElem/sec
format/json/baseline/fionn/large              1.00     80.1±2.76µs   503.6 MB/sec     1.16     93.1±2.10µs   433.0 MB/sec
format/scaling/ison/records/1000              1.20     47.4±2.06µs   850.3 MB/sec     1.00     39.6±0.67µs  1017.9 MB/sec
format/toml/parsing/structural/small          1.25    736.1±9.51ns   335.5 MB/sec     1.00   589.1±12.00ns   419.3 MB/sec
format/yaml/parsing/simd_scan/large           1.00     37.8±0.74µs   601.9 MB/sec     1.22     46.2±0.77µs   491.6 MB/sec
streaming/chunk_throughput/chunk_size/1000    1.22      4.2±0.12ms   282.0 MB/sec     1.00      3.5±0.02ms   342.9 MB/sec
streaming/chunk_throughput/chunk_size/2000    1.22      4.2±0.02ms   284.0 MB/sec     1.00      3.4±0.02ms   346.4 MB/sec
streaming/chunk_throughput/chunk_size/500     1.21      4.3±0.09ms   279.9 MB/sec     1.00      3.5±0.02ms   338.1 MB/sec
streaming/simd_comparison/simd_batch/1000     1.15    395.5±6.88µs   291.6 MB/sec     1.00    342.7±6.02µs   336.7 MB/sec
streaming/variable_lines/variable_1k          1.20    326.1±6.68µs   175.4 MB/sec     1.00    272.6±8.05µs   209.8 MB/sec

Threshold: 15% regression triggers warning

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Benchmarks compare this PR against main branch