feat(codegen): plumb-codegen crate + `plumb init --from ./`

[aram-devdocs]

Summary

Add a plumb-codegen crate that walks a source tree, infers
tokens from CSS / Tailwind / DTCG sources, and bootstraps a
best-effort plumb.toml. Wired through plumb init --from <path>.

Crate / area

plumb-cli — the plumb binary (only crate allowed stdout/stderr + anyhow)

Phase / batch

Part of #65 → Batch 7D (Codegen + watch + suggest-ignores).

Effort: L

PRD references

• PRD §12.3
• PRD §15.3

Acceptance criteria

• New plumb-codegen crate (layer: depends on plumb-core + plumb-config only).
• plumb init --from ./ produces a plausible plumb.toml on a real project.
• Deterministic output given deterministic inputs.

Dependencies

• None (first in batch; batch-level deps gate this).

Review gates

• 02-spec-reviewer
• 03-code-quality-reviewer
• 05-architecture-validator
• 04-test-runner

How to deliver

Dispatch /gh-issue <this-issue-number>. See the root AGENTS.md and the scoped plumb-cli/AGENTS.md for the invariants that apply.

Suggested delivery: gh-issue