Rename @webjsdev/ts-plugin to an intellisense-style name

[vivek7405]

Problem

@webjsdev/ts-plugin is named after its mechanism (a tsserver plugin), not its value (in-template IntelliSense: completions, diagnostics, hover, go-to-def). A name like @webjsdev/intellisense (or similar) reads as what it does for users and matches how it is surfaced in the editor plugins and docs.

Design / approach

Rename the package and the on-disk dir (packages/editors/ts-plugin -> packages/editors/intellisense), update every consumer: the VS Code extension + webjs.nvim bundle/vendor it; the scaffold pins it in node_modules + tsconfig.json plugins; backfill-changelog.js PACKAGES + PACKAGE_DIRS + PACKAGE_OLD_DIRS (keep the old changelog dir); the Dockerfile COPY path; CI; the start-work skill; all AGENTS.md references.

Prior art / context: #395 (CLOSED not-planned) rejected renaming ts-plugin -> @webjsdev/lsp because it is a tsserver plugin, NOT an LSP server, so lsp was a misnomer; the conclusion there was to KEEP ts-plugin. This issue revisits that with a DIFFERENT target name (intellisense), which describes the capability rather than mis-stating the protocol. Worth weighing: intellisense is Microsoft's brand term for completion (some baggage), and a published-package rename means a new npm name (the old name can be deprecated; webjs has no external users yet per the no-backward-compat stance, so a clean break is fine).

Since webjs ships no back-compat shims yet, prefer a clean rename over an alias.

Acceptance criteria

• Package renamed (npm name + dir), all consumers updated (vscode, nvim, scaffold, Dockerfile, CI, changelog tooling, docs).
• Old changelog/ts-plugin/ entries preserved; new entries land under the new name.
• A freshly scaffolded app still gets in-template intelligence; both editor plugins still bundle it.
• Decision recorded on the final name (reconciling refactor: rename @webjsdev/ts-plugin to @webjsdev/lsp #395).
• Tests + docs/AGENTS.md updated.