chore(cli): mutate only the lines a branch changed

[blafourcade]

🎯 What & why

Mutation-testing a change before pushing meant running a whole scope. The alternative was turning git diff -U0 into --mutate file:start-end ranges by hand, which was done five times in one session (#842).

🛠️ How it works

node scripts/run-mutation.mjs --changed [<base>]:

1. It diffs the working tree against its merge base with <base> (default origin/next). Uncommitted edits count.
2. It keeps only the lines added or changed under src/, as file:start-end ranges. A pure deletion adds no line, and a file outside src/ belongs to no scope.
3. It runs Stryker on those ranges only, and names each survivor with its file, line and mutator.

It holds no floor and passes no --incremental file, so it never moves a scope's gate or its cached results. Its reports go to reports/mutation/changed/.

Three pure functions carry the logic, and each has a test in mutation-covers-source.arch.test.ts's "the guard itself":

• changedRanges(diff);
• changedArgs(ranges);
• survivorsOf(report).

The report filing both modes share moved into fileReports. cli/aidd_docs/memory/testing.md has one line for the new mode.

🧪 How to verify

• Red first: the two new tests failed with changedRanges is not a function and survivorsOf is not a function.
• Real runs, with a throwaway HOME:

Command	Result
--changed HEAD	No line under src/ changed since HEAD: nothing to mutate.
--changed origin/next~1 (#836, a rename and one import)	no mutant on those lines, 1 changed range(s)
--changed origin/next~2 (adds #835)	score 97.6, 16 changed range(s); one survivor named: marketplace-sync-settings-use-case.ts:577 StringLiteral (the equivalent mutant #835 documents)
after all three	no incremental.json written

Local gates: typecheck, lint, knip, architecture 17/17 in the touched file.

⚠️ Heads-up

• No package script is added: mutation-covers-source.arch.test.ts requires every test:mutation:* script to be a declared scope, and --changed is not one.
• Untracked new files are not in git diff, so --changed does not see them until they are added.

🔗 Linked issue

Fixes #842

✅ I certify

• I DO CERTIFY I READ EACH LINE OF THE PULL REQUEST BECAUSE I AM A SOFTWARE ENGINEER, NOT A AI PUPPY.

🤖 Generated with Claude Code

https://claude.ai/code/session_011x4ms5qcGuZgYhCxfdHMUb