build: make Version dynamic via ldflags (const → var)

[nlenepveu]

Summary

The Makefile already sets LDFLAGS=-ldflags "-X main.Version=$(VERSION)" using git describe --tags, but this had no effect because Version was declared as a const — Go's linker -X flag only works on package-level vars.

As a result, every build (including official releases) always reports the hardcoded string (e.g. v0.27.5) regardless of the actual tag or commit.

Changes:

• cmd/agent-deck/main.go: const Version = "..." → var Version = "..." (one character change)
• Makefile: strip the leading v from git describe output so the injected value matches the existing "0.27.5" format and avoids a double-v prefix in the output

Before:

$ agent-deck --version
Agent Deck v0.27.5   # always, even on dirty/ahead builds

After:

$ agent-deck --version
Agent Deck v0.27.5             # clean tag build
Agent Deck v0.27.5-3-gabcdef0  # build ahead of tag

The default fallback value is unchanged — clean tag builds are unaffected.

Test plan

• make build && ./build/agent-deck --version on a clean tag checkout reports vX.Y.Z
• Same command on a commit ahead of the tag reports vX.Y.Z-N-gSHA
• make build with no git repo falls back to dev