feat(build): stamp commit, build number, and date into the binary

[Emasoft]

-version used to print a static string identical across every build — two binaries built a month apart were indistinguishable. Now it prints

late 2.0.0-rc.1 (build 1239, commit 2fe0e83, built 2026-09-25T13:12:11Z)

so every build is uniquely identifiable.

How it's stamped

• build number: git rev-list --count HEAD — deterministic, monotonic, works offline; reports unknown outside a git tree
• commit: short SHA from the same git probe
• date: RFC3339 build timestamp

The Makefile stamps all three via -ldflags -X, with a graceful fallback when git isn't available, and unknown pieces are omitted from the display instead of printed as noise.

Scope notes

• the info-bar version segment (compact v2.0.0-rc.1 · bN · <commit> form) rides with the info-bar PR — there's no info bar on main yet
• install-dev.sh stamping rides with the installer PR — there's no install-dev.sh on main

Testing

$ go test -count=1 -race ./...
ok  	late/cmd/late	1.535s
?  	late/cmd/mcp-run	[no test files]
ok  	late/internal/agent	2.495s
?  	late/internal/assets	[no test files]
ok  	late/internal/client	3.209s
ok  	late/internal/common	3.541s
ok  	late/internal/config	1.518s
ok  	late/internal/executor	10.428s
ok  	late/internal/git	3.492s
ok  	late/internal/mcp	4.231s
ok  	late/internal/orchestrator	2.489s
?  	late/internal/pathutil	[no test files]
ok  	late/internal/plugin	13.185s
ok  	late/internal/session	2.631s
ok  	late/internal/skill	3.373s
ok  	late/internal/tool	4.682s
ok  	late/internal/tool/ast	4.355s
ok  	late/internal/tui	9.788s

exit 0 — all 18 packages green (15 ok, 3 without test files).