feat(extension): implicit plugin mechanism via circleci-<name> PATH lookup

[michael-webster]

Summary

• Adds implicit extension support: any circleci-<name> binary in PATH is transparently invoked as circleci <name>, following git's plugin convention
• New internal/extension package handles PATH lookup, environment injection, and exit code propagation
• Only top-level unknown commands are intercepted — unknown subcommands within a group still produce the normal cobra error

How it works

When Cobra returns an unknown command "X" for "circleci" error, main.go intercepts it before printing and attempts to exec.LookPath("circleci-X"). If found, the binary is executed with:

• All args after the command name passed through
• CIRCLECI_TOKEN and CIRCLECI_HOST injected from the config file / env vars
• CIRCLECI_VCS_TYPE, CIRCLECI_PROJECT_USERNAME, CIRCLECI_PROJECT_REPONAME injected from git remote (best-effort, silent on failure)
• Full os.Environ() inherited so the extension gets everything the user had

If no matching binary exists, the original cobra error is shown unchanged.

Test plan

• task test — all 464 tests pass
• task check — lint clean (pre-existing license failure in cmd/.circleci/update_check.yml unrelated to this change)
• Manually tested with a fake circleci-hello extension: args, CIRCLECI_TOKEN, and CIRCLECI_HOST all injected correctly
• Manually tested circleci testsuite "ci tests" — dispatches to circleci-testsuite correctly, exit code propagated
• Unknown commands with no matching extension still show the original cobra error
• Unknown subcommands within a group (e.g. circleci pipeline foo) are not intercepted

Example running this:

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