Add comprehensive unit tests + CI

[cloudmanic]

Summary

Brings the project from one test file to nine, covering every source file. Driven by four parallel testing agents, one per package group.

Total statement coverage: 81.8%

Package	Coverage	Tests
internal/theme	100.0%	2
internal/filetree	96.2%	19
internal/editor	95.1%	67 (buffer 100%, tab ~98%, highlight ~76%)
internal/app	77.1%	93 (50 app + 35 modals + 8 pre-existing fileops)
internal/clipboard	30.0%	3 (limited by /dev/tty in CI)
internal/version	[no stmts]	3 (semver shape locked)

Plumbing

• Makefile — new test (with -race), test-short, and coverage targets. coverage writes coverage.out + a browsable coverage.html. clean nukes the coverage artifacts too.
• .github/workflows/test.yml — runs go test -race ./... on Linux + macOS for every push and every PR. Vets, verifies go.mod is tidy, builds, tests, and uploads the coverage profile as an artifact.
• .gitignore — adds /coverage.out and /coverage.html.
• CLAUDE.md — replaces the one-line "Tests" section with the full bar: every source file gets a _test.go in the same package, bug fixes need a regression test, t.Skip is reserved for hard environment limits (e.g. /dev/tty).
• README.md — notes the new make targets and that CI runs tests on every push / PR.

What's not covered

The TUI's interactive bits stay untested:

• App.New / App.Run (real terminal screen + goroutine wiring)
• startTreeRefresh / reconcileOpenTabsWithDisk (only fires on the background tick)
• handleAutoScroll mid-drag (needs a live drag plus an autoScrollEvent)
• clipboard.CopyToSystem end-to-end on machines without /dev/tty (the encoding contract is still locked down via a byte-mirror test)

These are the "test from the TUI manually" paths the architecture explicitly carves out. Everything else is exercised via tcell.SimulationScreen + t.TempDir.

Test plan

• CI run is green on this PR (Linux + macOS)
• make test locally — green, no flakes
• make coverage locally — opens coverage.html and shows the per-file numbers above
• go test -race ./... — no race detector findings
• Spot-check a few new test files for the doc-comment style described in CLAUDE.md