feat(web): idx website and user docs (Astro Starlight)

Landing page plus docs for install, quickstart, config, the stocks and
ownership CLI, output and exit codes, the MCP server and per-agent setup,
agent usage, and an agent-led install page (agents/install.md) that a
coding agent follows from a one-line prompt.

- Every docs page is also served as Markdown at <path>.md; llms.txt and
  llms-full.txt come from starlight-llms-txt; a SKILL.md for skill-based
  harnesses is under /skills/idx/.
- The CLI reference and MCP tools pages are generated from idx --help and
  the tools golden; a new web-generated flake check fails when they drift.
- nix develop .#web provides bun; Cloudflare Pages builds web/ on pushes.
- Package homepage metadata points at https://idx.rasyidanaf.com.
- .cloudflare/ (the cf CLI's account cache) is git-ignored.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/content/docs/cli/output.md

@@ -0,0 +1,53 @@
+---
+title: Output and exit codes
+description: Tables, JSON, errors and exit codes; the contract scripts and agents rely on.
+---
+
+## Formats
+
+| `-o` | Prints |
+| --- | --- |
+| `auto` (default) | a table when stdout is a terminal, compact JSON otherwise |
+| `table` | a table |
+| `json` | one JSON document |
+| `jsonl` | one JSON value per line (one per array item) |
+
+Set the default with `IDX_OUTPUT` or `general.output`. Colour is off whenever
+stdout is not a terminal, and with `--no-color`.
+
+## The contract
+
+- stdout carries only data; warnings, progress and errors go to stderr.
+- On failure stdout is empty.
+- In JSON modes an error is one JSON object on stderr:
+
+  ```json
+  {"error":true,"code":"INVALIDINPUT","message":"invalid input: invalid ticker symbol '../x'"}
+  ```
+
+## Exit codes
+
+| Exit | Meaning | What to do |
+| --- | --- | --- |
+| `0` | success | |
+| `2` | invalid input or config | fix the command |
+| `3` | not found, no data, or no cached data offline | check the ticker or name; go online |
+| `4` | upstream unavailable | retry later |
+| `5` | unsupported | use another command or provider |
+| `1` | anything else | read the message |
+
+## Pipes
+
+```bash
+# names and prices
+idx stocks quote BBCA BBRI | jq -r '.[] | "\(.symbol) \(.price)"'
+
+# the last five daily closes
+idx -o jsonl stocks history BBCA --period 1y | tail -5
+
+# symbols from stdin
+idx stocks screen --filter high-dividend | jq -r '.[].symbol' | idx stocks compare -
+```
+
+`stocks quote` and `stocks compare` read symbols from stdin when given `-`,
+separated by whitespace or commas.