[codex] Milestone 9 checkpoint

[OneLastStop529]

Summary

This checkpoint closes out Milestone 9 by making the CLI follow a single Redis-backed runtime model and documenting that path clearly.

Changes in this PR:

• default worker, enqueue, result, and dlq to Redis-backed state for normal CLI usage
• add env-driven Redis CLI configuration and CLI tests for the new defaults
• add make install and repo-local zsh completion support for the taskforge command
• let taskforge result -id resolve a unique task ID prefix
• update README, milestones, and architecture docs to mark Milestone 9 as implemented
• ignore generated local artifacts such as bin/ and .direnv/

Why

The project previously supported Redis-backed multi-process execution, but the default CLI path still led users into isolated in-memory behavior. That created ambiguity around the intended runtime model and made the out-of-the-box flow harder to use.

This checkpoint makes Redis the default CLI path, keeps explicit in-memory escape hatches for demo and test flows, and tightens the local developer ergonomics around install and completion.

Validation

• env GOCACHE=/tmp/taskforge-gocache go test ./...

Notes

• The repo-local zsh completion loader is explicit and does not rely on direnv mutating FPATH.
• taskforge result -id <prefix> now requires the prefix to resolve uniquely; ambiguous prefixes return an error.