ci: move perf gate and changelog onto the soothfast action

[Verdenroz]

Description

The perf gate and the changelog regeneration were two hand-copied workflows, and the changelog one needed the GitHub App private key in this repo's secrets to mint its token. Both now run as one soothfast job in ci.yml through the Verdenroz/soothfast action, which proves its identity over OIDC and holds no secrets. Pull requests get the gate result as a soothfast-bot comment. Pushes to main refresh the baseline and land CHANGELOG.md as a bot/soothfast-update PR that merges itself.

Changes

• Bumped soothfast to 0.3.1.
• Removed gate.yml and changelog.yml.
• Added the soothfast job to ci.yml, pinned to the v0.3.1 commit. changelog-packages keeps scry-server in the changelog diff while the gate measures only scry-core.
• Replaced the unpinned cargo install cargo-soothfast in docs.yml with the action in install-only mode, so the CLI matches Cargo.lock.
• Skipped the job on bot-authored pushes so the bot's own merge does not regenerate again.
• Updated CLAUDE.md.

Testing

• cargo check -p scry-core --benches --tests compiles against soothfast 0.3.1.
• The soothfast-bot environment exists with no branch policy and the App is installed. The main ruleset's required check swaps from gate to soothfast once this PR's checks appear.

[soothfast-bot]

soothfast gate

scry-core

ok    scry_core::bench_chunk_rust callgrind_ir 75706022.0 -> 75605560.0 (-0.1%)
ok    scry_core::bench_chunk_rust walltime_median_ns 7172646.0 -> 7196624.0 (+0.3%)
ok    scry_core::bench_chunk_rust allocs 3628 -> 3628 (allowed <= 3809)
ok    scry_core::bench_chunk_rust alloc_bytes 793931 -> 793931 (allowed <= 833627)
ok    scry_core::bench_dense_search instructions 88391909.0 -> 88391909.0 (+0.0%)
ok    scry_core::bench_dense_search callgrind_ir 88391909.0 -> 88391909.0 (+0.0%)
ok    scry_core::bench_dense_search walltime_median_ns 8404854.0 -> 8389993.0 (-0.2%)
ok    scry_core::bench_dense_search allocs 11 -> 11 (allowed <= 11)
ok    scry_core::bench_dense_search alloc_bytes 16274 -> 16274 (allowed <= 17087)
NOTE  scry_core::bench_expand_symbols: code changed since reference
ok    scry_core::bench_expand_symbols instructions 31088.0 -> 30459.0 (-2.0%)
ok    scry_core::bench_expand_symbols callgrind_ir 31088.0 -> 30459.0 (-2.0%)
ok    scry_core::bench_expand_symbols walltime_median_ns 2555.9 -> 2562.5 (+0.3%)
ok    scry_core::bench_expand_symbols allocs 55 -> 55 (allowed <= 57)
ok    scry_core::bench_expand_symbols alloc_bytes 2366 -> 2366 (allowed <= 2484)
ok    scry_core::bench_hash_bytes instructions 917595.0 -> 917595.0 (+0.0%)
ok    scry_core::bench_hash_bytes callgrind_ir 917595.0 -> 917595.0 (+0.0%)
ok    scry_core::bench_hash_bytes walltime_median_ns 75681.2 -> 75961.3 (+0.4%)
ok    scry_core::bench_hash_bytes allocs 0 -> 0 (allowed <= 0)
ok    scry_core::bench_hash_bytes alloc_bytes 0 -> 0 (allowed <= 0)
ok    scry_core::bench_lexical_search instructions 157758928.0 -> 157758928.0 (+0.0%)
ok    scry_core::bench_lexical_search callgrind_ir 157758928.0 -> 157758928.0 (+0.0%)
ok    scry_core::bench_lexical_search walltime_median_ns 16244811.0 -> 16233815.0 (-0.1%)
ok    scry_core::bench_lexical_search allocs 34 -> 34 (allowed <= 35)
ok    scry_core::bench_lexical_search alloc_bytes 2467 -> 2467 (allowed <= 2590)
ok    scry_core::bench_normalize_remote_url instructions 2824.0 -> 2824.0 (+0.0%)
ok    scry_core::bench_normalize_remote_url callgrind_ir 2824.0 -> 2824.0 (+0.0%)
ok    scry_core::bench_normalize_remote_url walltime_median_ns 232.4 -> 231.1 (-0.6%)
ok    scry_core::bench_normalize_remote_url allocs 5 -> 5 (allowed <= 5)
ok    scry_core::bench_normalize_remote_url alloc_bytes 116 -> 116 (allowed <= 121)
ok    scry_core::bench_search_with_vector instructions 384605295.0 -> 384609645.0 (+0.0%)
ok    scry_core::bench_search_with_vector callgrind_ir 384605295.0 -> 384609645.0 (+0.0%)
ok    scry_core::bench_search_with_vector walltime_median_ns 37784295.0 -> 37728480.0 (-0.1%)
ok    scry_core::bench_search_with_vector allocs 1286 -> 1286 (allowed <= 1350)
ok    scry_core::bench_search_with_vector alloc_bytes 169014 -> 169014 (allowed <= 177464)
ok    scry_core::bench_chunk_rust assert complexity: claimed O(n); growth drift x1.02 over sizes [64, 256, 1024] (limit x2.5)
ok    scry_core::bench_hash_bytes assert alloc: allocs 0 <= 0
ok    scry_core::bench_hash_bytes assert complexity: claimed O(n); growth drift x0.98 over sizes [4096, 65536, 1048576] (limit x2.5)
ok    scry_core::bench_normalize_remote_url assert alloc: allocs 5 <= 5
gate: passed (7 item(s))