review,diff: add --include glob flag to scope reviews to matching files

[selyafi]

What

Adds --include <pattern> (repeatable) — the allowlist counterpart of --exclude — so a review can be scoped to one area of a large diff before the prompt is built.

How

• Engine (internal/diff/include.go): diff.Include() keeps only files matching at least one gitignore-lite pattern and drops the rest from both the parsed file list and the rebuilt raw diff. Reuses the shared fileMatchesAny / matchPattern / splitRawSegments helpers, so pattern rules (trailing / = dir tree at any depth, no / = basename glob, else full-path glob), the rename old/new-path union, and the positional raw-segment safety check are identical to Exclude. No patterns = no-op ("review everything").
• Wiring (internal/app/review.go): registered in registerModelFlowFlags, so it lands on review, inbox, and bare diffsmith. applyIncludes() runs before applyExcludes() on both the bypass (--print-prompt/--dry-run) and TUI paths, so --exclude carves exceptions out of the included set (--include 'internal/' --exclude 'internal/gen/').
• No silent omissions: the narrowing is surfaced in the run summary (or stderr on the bypass path); an --include that matches no changed file is a clean pre-model error, and malformed globs fail up front.
• Adapters: the three over-budget hints now name --include/--exclude.
• Docs: README paragraph + new ## Unreleased CHANGELOG section.

Tests

TDD throughout — 10 new diff.Include unit tests + 5 new command-level --include tests (including include→exclude composition). Each watched fail before implementing. Full go test ./..., go vet ./..., and go build ./... green.

Closes diffsmith-dko.

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