test: cover slack keychain metadata

[rianjs]

Summary

• bump cli-common to the immutable pseudo-version containing test: cover macOS keychain metadata cli-common#64
• add a Darwin+cgo gated empirical test for slck Keychain metadata
• seed stale metadata for both bot_token and user_token, then verify the adapter overwrite path repairs concrete labels/descriptions

Verification

• go list -m github.com/open-cli-collective/cli-common
• go mod tidy -diff
• go test ./internal/keychain
• SLCK_KEYCHAIN_METADATA_TEST=1 go test ./internal/keychain -run TestKeychainMetadataGated -count=1 -v
• go test ./...
• make test
• make verify

Closes #188

[rianjs]

Findings

• Minor: The only new regression guard is opt-in (SLCK_KEYCHAIN_METADATA_TEST=1), so normal go test ./... / CI runs will skip it. That makes this a manual proof, not automated coverage, and a future metadata regression could still merge unless you also wire a Darwin job or an always-on assertion. internal/keychain/keychain_metadata_darwin_test.go
• Minor: The test only exercises the stale-metadata overwrite path. It never verifies the first-time SetBotToken/SetUserToken write path on an absent item, so a bug where initial creation stops writing concrete labels/descriptions would still pass. internal/keychain/keychain_metadata_darwin_test.go

[monit-reviewer]

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[monit-reviewer]

Automated PR Review

Reviewed commit: 4ccdeb1

Manually approved via CLI.