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/agents/index.md

@@ -0,0 +1,60 @@
+---
+title: Using idx from an agent
+description: How an AI agent should call idx, through MCP or the CLI.
+---
+
+An agent can reach idx two ways:
+
+- **MCP** ([setup](/mcp/setup/)): typed tools with input schemas, the best
+  fit for chat-style agents.
+- **The CLI**: for agents that run shell commands. No setup beyond
+  installing `idx`; output is JSON automatically when captured.
+
+## Using the CLI
+
+What an agent needs to know:
+
+- Output is JSON whenever stdout is not a terminal, so capture it and parse
+  it; no flag is needed. Use `-o jsonl` for long lists.
+- On failure stdout is empty and stderr holds one JSON object,
+  `{"error": true, "code": "...", "message": "..."}`.
+- Exit codes say what to do next: `2` fix the input, `3` not found (or no
+  cached data offline), `4` provider unavailable, retry later, `5`
+  unsupported, `1` other.
+- Tickers are IDX codes (`BBCA`); `.JK` is optional. Several tickers can be
+  passed at once to `stocks quote` and `stocks compare`.
+- Ownership queries need `idx ownership sync` once; after that they run
+  offline. `idx -o json ownership releases` shows which months exist.
+
+## Recipes
+
+```bash
+# price and day change for several stocks
+idx stocks quote BBCA BBRI BMRI | jq '.[] | {symbol, price, change_pct}'
+
+# largest holders and concentration
+idx ownership ticker BBCA | jq '{holders: [.holders[] | {name, pct: (.percentage_bps / 100)}], concentration}'
+
+# what one investor holds
+idx ownership entity "ANTHONI SALIM"
+
+# holder changes between the two latest months
+idx -o json ownership releases | jq -r '.[0:2][].as_of_date'
+idx ownership changes --from 2026-08-31 --to 2026-09-30
+```
+
+Percentages in ownership output are basis points (`5494` = 54.94%).
+
+## Skill file
+
+For harnesses that load skills (Claude Code, pi, opencode, and others),
+[`SKILL.md`](/skills/idx/SKILL.md) packages these rules so the agent uses the
+CLI correctly without MCP. Save it as `idx/SKILL.md` in your harness's skills
+directory, for example `~/.claude/skills/idx/SKILL.md`.
+
+## Machine-readable docs
+
+- [`/llms.txt`](/llms.txt): an index of these docs for language models.
+- [`/llms-full.txt`](/llms-full.txt): every page in one file.
+- Any page as Markdown: add `.md` to its path, for example
+  [`/agents/install.md`](/agents/install.md).