fix: correct Linear API key settings URL (#134)

## Summary

- Fixes the URL shown in the `LINEAR_API_KEY is not set` error message:
`https://linear.app/settings/api` →
`https://linear.app/settings/account/security`
- Updates the matching test assertion in `missing_api_key_test.exs`
- Updates `Readme.adoc` setup instructions with the correct URL

Linear moved API key management from `/settings/api` to **Settings >
Account > Security & Access** (`/settings/account/security`). The old
URL was sending users to a non-existent page.

Closes CRY-52.

## Test plan

- [ ] `mix test test/linear_cli/cli/missing_api_key_test.exs` passes
(verified locally)
- [ ] Full test suite: 248/254 pass (6 pre-existing `LinearCli.GitTest`
failures unrelated to this change)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Author: bougyman

Readme.adoc

@@ -129,7 +129,7 @@ $ ./burrito_out/lc_<target> --version
 === Configuration
 
 You must set the `LINEAR_API_KEY` environment variable to your Linear API key.
-You can find your API key in your https://linear.app/settings/api[Linear Settings].
+You can find your API key in your https://linear.app/settings/account/security[Linear Settings].
 
 === Commands
 

app/lib/linear_cli/cli.ex

@@ -311,7 +311,12 @@ defmodule LinearCli.CLI do
          halt
        ) do
     IO.puts(:stderr, "LINEAR_API_KEY is not set.")
-    IO.puts(:stderr, "Set it to your Linear API key - see https://linear.app/settings/api")
+
+    IO.puts(
+      :stderr,
+      "Set it to your Linear API key - see https://linear.app/settings/account/security"
+    )
+
     IO.puts(:stderr, "** Missing configuration, cannot continue **")
     maybe_print_backtrace(debug)
     halt.(78)

app/test/linear_cli/cli/missing_api_key_test.exs

@@ -16,7 +16,7 @@ defmodule LinearCli.CLI.MissingApiKeyTest do
 
     assert_received {:halted, 78}
     assert output =~ "LINEAR_API_KEY is not set."
-    assert output =~ "https://linear.app/settings/api"
+    assert output =~ "https://linear.app/settings/account/security"
     refute output =~ "What the heck is this?"
     refute output =~ "Ash.Error"
   end