feat(alias): user-level shorthand names for module sources

[rickliujh]

Shortens long invocations. Instead of:

loom run git@github.com:foo/bar.git//modules/svc -p foo=bar -p something=anotherthing

you get:

loom alias add bar git@github.com:foo/bar.git//modules/svc -p foo=bar -p something=anotherthing
loom :bar                  # run it
loom :bar -p foo=other     # override a param
loom diff :bar             # preview it
loom alias list | remove

An alias maps a short name to a module source plus a set of default parameters.

Design notes

A leading : marks a reference. The sigil makes the alias namespace disjoint from the subcommand namespace, which is what lets loom :bar dispatch at top level: no bare name can shadow a subcommand, so a typo like loom rnu still gets cobra's usual unknown command error and "Did you mean: run" suggestion.

Top-level dispatch is an argv rewrite (loom :bar … → loom run :bar …), not a root-level handler. A root handler was tried first and fails with unknown shorthand flag: 'p' — the run flags live on runCmd, so root dispatch would have to redeclare every one of them. The rewrite inherits the whole flag set for free.

Aliases are user-level only. One file at $LOOM_CONFIG_DIR/aliases.yaml, else <user config dir>/loom/aliases.yaml. No repo-local file and no upward search, so changing branch or directory can never change what an alias resolves to.

Aliases are never module configuration. Resolution happens at the CLI boundary; module.ResolveSource stays alias-blind, so spec.modules[].source cannot depend on whichever aliases an operator happens to have defined. Modules stay portable.

Alias params are the lowest-precedence source, beneath --params-file and -p. Module spec.params defaults still apply beneath all three.

Unknown aliases don't fall through to the git-URL path, so a mistyped alias reports a mistyped alias rather than a failed clone.

Scope

loom run and loom diff accept :<name>. loom validate and loom bulk deliberately do not: neither consumes alias params, validate resolves no remote sources at all today, and bulk embeds the source it is given into the module it generates — where a reference would resolve for nobody else. Extending them is additive.

Single-segment names only. The alias file is a flat map[name]Def, so multi-segment names (loom :svc onboard) remain a lookup-loop change rather than a schema change.

Testing

• specs/alias.md defines AL1–AL13; spectrace passes, so every rule has a covering test.
• Full suite green (go build, go vet, go test ./...).
• Verified end-to-end against the built binary: add → list → loom :bar renders the templated output; -p overrides the alias param; loom :typo reports unknown alias "typo" (looked in …) with no clone attempt; loom : errors on the empty name; loom rnu still reports an unknown command; loom diff :bar previews.
• gofmt flags only files that were already unformatted on main (pkg/generate, pkg/action, pkg/llm); none touched here.

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