fix: drop the soothfast bot's own commits from the changelog

[Verdenroz]

What changed

• Read the commit author alongside the subject in report changelog's git log call, and dropped every commit the soothfast bot wrote before the subjects reach the parser.
• Added [changelog] bot-author in soothfast.toml and a --bot-author NAME flag, for a repo that renamed the action's bot-slug. The flag wins, then the table, then the shipped soothfast-bot[bot].
• Documented the filter in docs/reports.md, including that it drops exactly one author and leaves every other bot in place.

Why

The only guard against bot noise was a list of this repo's own regeneration subjects, which no adopting repo matches. A consumer lands its regenerated artifacts under the title the action sets, so those commits kept showing up in its own Unreleased section under Internal. Author identity is what actually survives a squash merge, and the bot always commits as one fixed name. The filter names that one author and nothing else: dependabot[bot] writes an entire Dependencies section, and a rule matching every [bot] author would delete it without a trace.

How was this tested

• cargo test -p cargo-soothfast -p soothfast-report: 233 passed, 1 failed in cargo-soothfast; 34 passed, 1 ignored in soothfast-report.
• The one failure is invoke::tests::a_plain_directory_in_the_repo_is_not_a_worktree_root, which is cwd-dependent and fails from any linked worktree. This branch does not touch invoke.rs.
• cargo fmt --all -- --check: clean.
• cargo clippy --workspace --all-targets -- -D warnings: clean.
• Eight new unit tests cover the filter and the precedence, including one asserting a dependabot[bot] commit survives the filter.
• make check and make gate BASE=master were not run.

Checklist

• make check passes (fmt, clippy -D warnings, cargo test --workspace)
• make gate BASE=master passes, or any intentional cost change is explained above
• Tests added/updated for new behavior (not required for docs/CI-only changes)
• Public API items have /// doc comments
• No new dependency, or its justification is included above (see Dependency Policy)
• Docs (README.md, docs/, soothfast:bind/soothfast:claim markers) updated if behavior changed

[soothfast-bot]

soothfast gate

gate: measuring merge-base of origin/master in worktree (interleaved rounds)
gate: bench binaries identical (code and data match) — no measurable change possible
gate: gating backend = walltime
gate: build=08f03a noise_floor=0.07% thresholds: instructions +5% ir +5% walltime +10.0% alloc/size +5% polls/wakes +5%
ok    soothfast_site::bench_highlight walltime_median_ns 3123235.0 -> 3123235.0 (+0.0%)
ok    soothfast_site::bench_highlight allocs 88069 -> 88069 (allowed <= 92472)
ok    soothfast_site::bench_highlight alloc_bytes 2079336 -> 2079336 (allowed <= 2183302)
ok    soothfast_site::bench_md_render walltime_median_ns 3067862.0 -> 3067862.0 (+0.0%)
ok    soothfast_site::bench_md_render allocs 68118 -> 68118 (allowed <= 71523)
ok    soothfast_site::bench_md_render alloc_bytes 3502526 -> 3502526 (allowed <= 3677652)
ok    soothfast_site::bench_highlight assert complexity: claimed O(n); growth drift x1.19 over sizes [128, 512, 2048] (limit x2.5)
ok    soothfast_site::bench_md_render assert complexity: claimed O(n); growth drift x1.02 over sizes [256, 1024, 4096] (limit x2.5)
gate: passed (2 item(s))