fix: skip update when already on latest version (#142)

[dLo999]

Closes #142

Summary

Adds a version check to the update command that skips the download/install when the binary is already on the latest (or newer) version. Uses the GitHub /releases/latest redirect header to determine the latest tag — no JSON library needed in cli.c.

Changes

• src/cli/cli.c: Added fetch_latest_tag() helper that follows the GitHub releases redirect via curl -sfI and extracts the tag from the Location: header
• src/cli/cli.c: Added version comparison block at the start of cbm_cmd_update() using the existing cbm_compare_versions() infrastructure
• Added --force flag to bypass the version check when needed
• Network failures produce a warning but don't block the update (graceful degradation)

Behavior

Scenario	Output
Already on latest	Already up to date (v0.5.7). → exit 0
Ahead of latest (dev build)	Already up to date (v0.5.8-dev, ahead of latest v0.5.7). → exit 0
Newer available	Update available: v0.5.6 -> v0.5.7 → continues with download
Network failure	warning: could not check latest version → continues with download
--force flag	Skips check entirely → always downloads
CBM_DOWNLOAD_URL set	Skips check (testing path)

Test results

Build: Compiles cleanly on macOS (Apple Clang, arm64) with make -f Makefile.cbm cbm — no extra dependencies needed (zlib ships with Xcode CLT).

Test suite: 2741 tests passed, 0 regressions.

Manual verification (all with --dry-run --standard):

Test	Version	Result
Already on latest	CBM_VERSION=v0.5.7 (== latest)	Prints Already up to date (v0.5.7)., exits 0 immediately
Update available	CBM_VERSION=v0.5.6 (< latest)	Prints Update available: v0.5.6 -> v0.5.7, continues to download
Force bypass	CBM_VERSION=v0.5.7 + --force	Skips version check, proceeds directly to download

Generated with agent-team via /issue

[dLo999]

Review Summary

Adds a version check to cbm update that skips download/install when already at latest version. Uses curl redirect header parsing to fetch the latest GitHub release tag, compares with existing cbm_compare_versions() infrastructure. Graceful degradation on network failure.

Findings

• [nit] src/cli/cli.c:2975 — fetch_latest_tag() uses cbm_popen() with hardcoded URL via shell. NOLINTNEXTLINE bypass is acceptable but should document why for maintainability.
• [warning] src/cli/cli.c:3024 — getenv("CBM_DOWNLOAD_URL") concurrency bypass is safe in CLI context but comment should note this is only safe because update is never called from multi-threaded code.
• [question] src/cli/cli.c:3050 — --force flag is not visible in help text in this diff. Is it documented elsewhere (e.g. cbm update --help)?
• [nit] src/cli/cli.c:2993-3001 — Silent failure on malformed Location header is safe (returns NULL) but a debug warning could aid diagnosis.

CI Status

No CI checks on fork branch. Local testing: 2741 tests passed, 0 regressions (macOS Apple Clang arm64). Manual verification of all 3 scenarios (up-to-date, update available, --force bypass) confirmed working.

Verdict

APPROVE — Implementation is correct, memory-safe, and handles edge cases well. Minor documentation gaps only.