Update outline to v0.1.8

[andrew]

Update outline to v0.1.8 so brief outline inherits content-based binary classification from magic.

Add a command-level fixture for ASCII PDF content that the previous classifier treated as text and a fixed outline benchmark. Interleaved measurements on this repository show startup at +1.05%, a normal scan at +0.16%, and outline at -0.47%. The linked binary grows by 33,776 bytes, while benchmark allocations fall from 1,929 to 1,919.

[Copilot]

Pull request overview

Updates the brief outline command to use github.com/git-pkgs/outline v0.1.8 (which now inherits content-based binary classification via magic) and adds command-level tests/benchmarks to lock in the new behavior.

Changes:

• Bump github.com/git-pkgs/outline to v0.1.8 (and pull in github.com/git-pkgs/magic indirectly).
• Refactor runOutline to accept an io.Writer to make output testable.
• Add a regression test for ASCII PDF content being detected/skipped as binary and add an outline packing benchmark.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 2 comments.

File	Description
go.mod	Bumps outline to v0.1.8 and adds magic as an indirect dependency.
go.sum	Updates dependency checksums for outline v0.1.8 and magic v0.1.0.
cmd/brief/outline.go	Routes outline output through an injected io.Writer instead of hard-coding os.Stdout.
cmd/brief/outline_test.go	Adds regression test for binary skipping and a benchmark for outline.Pack.

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