docs: drop AI-slop wording from README

[dobby-coder]

Sweeps the repo for AI-slop writing tells per the rules in the memory repo. Findings were small.

Changes

README.md:

• L7: replaced "In the PostGuard ecosystem" with "Within PostGuard". "ecosystem" is on the avoid list (overstretched metaphor).
• L46: tightened the license paragraph. Dropped "This is the standard Rust ecosystem licensing arrangement and is declared as ..." — that sentence was meta-commentary plus another use of "ecosystem". Now just states the Cargo.toml declaration.

What was checked but left alone

• src/lib.rs, src/gg.rs, benches/bench.rs: doc comments and code comments are terse and technical. No tells worth touching.
• No UI in this crate, so the design tells do not apply.
• Grepped the high-signal vocabulary (delve, leverage, harness, robust, comprehensive, seamless, navigate, foster, unlock, realm, landscape, tapestry, pivotal, etc.) — only the two "ecosystem" hits surfaced.

Verification

• cargo build clean.
• cargo test — 15 unit tests + 1 doc test pass.

Reviewer quickstart

git fetch origin && git checkout chore/ai-slop-sweep && cargo test

Closes #11
Refs encryption4all/dobby#44