docs: rewrite README around differentiators

[jmgilman]

Summary

Rewrites the README to be briefer and human-parseable, leading with what sets CodeMode apart instead of dense contract language.

• Intro now says plainly what the library does: agents write Starlark programs against registered Go functions instead of chaining tool calls.
• New comparison list covers the differentiators: one-round-trip composition with intermediate data outside the context window, type-derived schemas that cannot drift, per-deployment capability disabling, fail-closed per-call authorization with the optional Rego adapter, and fresh worker processes.
• Install section switches from "no release" to @v0.1.0 ahead of the upcoming first release.
• Security boundary compressed to the load-bearing contract (fresh killable worker, bind -> authorize -> dispatch, host-owned identity, budgets are not OS quotas); the choreography detail stays in the security model and reference docs, which already cover it.
• Dropped the SearchTerms digression and duplicate ServeWorkerAndExit/TestMain prose; the tutorial and reference own those.

No behavior or doc-content claims were added; every listed mechanism is verified shipped behavior.

Validation

• Claims cross-checked against the current surface (three tools, ranked search, worker execution, Rego adapter, stock resolvers) as exercised in the live smoke run at 344b79b.
• Note: go get github.com/meigma/codemode@v0.1.0 resolves only after the v0.1.0 tag is published; merge with or after the release.