docs: polish user-facing text for clarity and consistency

[NikolayS]

This PR is a copy-editing pass over the user-facing text in the repository. Every change is prose only: grammar, American-English spelling, sentence-case headings, terminology, and clearer opening lines for search snippets and LLM summaries. No code, commands, SQL, config values, or numbers were touched.

What changed

• README.md: made the opening line self-contained for search snippets and LLM summaries ("rpg is a psql-compatible Postgres terminal written in Rust..."); gave the AI assistant and /text2sql intros explicit subjects and active voice; tidied the psql-compatibility wording ("diffed — a test passes only if they are identical", "The skips are due to...").
• docs/WASM.md: sentence-cased the H1 and section headings (quick start, what works, known limitations, output routing, WebSocket proxy) and the in-text link, kept the anchor unchanged; replaced "— e.g." with ", for example"; added missing articles.
• docs/psql-compat.md: small framing fixes ("REPL" -> "the REPL", "output byte-identical to psql's", comma before "etc.").
• docs/COMMANDS.md and docs/GLOSSARY.md: British-to-American spelling fixes ("behaviour" -> "behavior", "visualiser" -> "visualizer"); command tokens and glossary terms untouched.
• CONTRIBUTING.md: fixed a sentence fragment in the Integration tests section.
• CHANGELOG.md: two American-English spelling fixes within existing entry prose ("behaviour" -> "behavior", "in favour of" -> "in favor of").
• src/main.rs: one grammar fix in a --generate-wrappers doc-comment ("e.g." -> "e.g.,"). No runtime string, flag, default, or logic changed; verified no test asserts on it.
• wasm/index.html: one on-screen terminal message reworded ("Press Connect to start." -> "Click Connect to start."); ANSI escape codes preserved, no test asserts on it.

Not changed

No code, commands, SQL, config values, flags, identifiers, URLs, version tags, benchmark numbers (222 of 232, 10 skipped), badges, demo GIF paths, tables, or quoted psql/rpg output were changed. Pre-existing em dashes were preserved, not churned. For the two source files (src/main.rs, wasm/index.html) only comment/message text changed, and grep confirmed no test asserts on those strings.

Relates to #838

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Note: commit eec6d95 (style(test): satisfy clippy assert_eq lint) is a test-only change required to get pre-existing clippy passing under -D warnings. Two manual_assert_eq denials in test code (src/config.rs, src/ash/sampler.rs) were surfaced by the PR merge commit; they are unrelated to the prose polish.