fix(cli): stop sending the retired hidden input to tabs/windows

[Athul Nambiar (athul-22)]

Summary

browseros-cli open is broken on every code path because the CLI still sends the hidden input that the server retired.

Both tool schemas are #[serde(deny_unknown_fields)] and pages.newPage rejects the option explicitly, so the call fails before a tab is ever opened:

CLI path	sends	server response
open → tabs action=new	"hidden": false	Invalid arguments for tabs: hidden: unknown field `hidden`
open --window N → browser.pages.newPage(url, { hidden, … })	hidden: false	pages.newPage: hidden is no longer supported
window create → windows action=create	"hidden": false	Invalid arguments for windows: hidden: unknown field `hidden`

The retirement is intentional and pinned by the server's own tests — retired_hidden_inputs_are_rejected and tab_and_window_schemas_omit_hidden_controls in crates/browseros-mcp/src/tests.rs. Only the Go CLI was left behind.

Fix

Drop the hidden argument and the --hidden flag from open and window create, per the rule in apps/cli/CLAUDE.md:

MCP tool names are the contract. A command only shapes args; the server owns the tool. Keep command tool names and arg keys in sync with the compact MCP surface.

This also fixes #2162

open --json was reported as returning {"url":"…"} with no page. That was a symptom, not a second bug — tabs action=new already returns { "page": <id> } (tools/tabs.rs), and openResult already maps it (cmd/open.go). The field was missing only because the call never succeeded. With hidden gone, open --json emits the documented shape again:

{"page":123,"url":"https://example.com"}

which is what cmd/llm_txt.md tells agents to rely on:

page=$(browseros-cli open --json https://example.com | jq -r .page)
browseros-cli -p "$page" snapshot

Changes

• cmd/open.go — remove hidden from openTabsToolArgs and openInWindowCode; drop the --hidden flag
• cmd/window.go — remove hidden from window create; drop the --hidden flag
• cmd/tool_mapping_test.go — update the two mapping tests, plus a guard asserting openInWindowCode never re-introduces hidden
• cmd/llm_txt.md, README.md, CHANGELOG.md — drop the stale --hidden references

Verification

Per apps/cli/CLAUDE.md, run from apps/cli/:

gofmt -l .     # clean
go vet ./...   # clean
go build ./... # ok
go test ./...  # all packages pass

--hidden is gone from both help outputs:

$ browseros-cli open --help
Flags:
      --bg           Open in background
  -h, --help         help for open
      --window int   Window ID to open in

Noted, not addressed

browseros-cli window activate <id> (cmd/window.go) sends action: "activate", but WindowsAction intentionally omits Activate ("so MCP callers cannot steal window focus"), so that subcommand is dead too. Removing a user-facing command felt like a maintainer call rather than part of this fix — happy to fold it in or open a separate issue, whichever you prefer.

Fixes #2161
Fixes #2162

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[Athul Nambiar (athul-22)]

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