fix: give a clear message and exit 78 when LINEAR_API_KEY is missing

[bougyman]

Summary

Closes #74. A missing LINEAR_API_KEY previously surfaced as a raw %Ash.Error.Unknown{} dump reaching the generic catch-all handler ("What the heck is this?" + a full Ash stacktrace) - the Ash manual actions wrapping LinearCli.Api.call/2's {:error, :missing_api_key} produced a shape that didn't match either existing specific handle_error/3 clause.

Adds a clause matching that exact wrapped shape (Ash stringifies the original reason into UnknownError's :error field rather than preserving the atom - verified directly, not guessed) and prints a clear, actionable message instead, with sysexits.h's EX_CONFIG (78) since this is a configuration problem, not a crash. Plain pattern match, no guard, so it degrades gracefully to the existing catch-all if Ash's wrapping format ever changes rather than introducing a new failure mode.

Test plan

• Reproduced the original bug exactly (env -u LINEAR_API_KEY mix run -e 'LinearCli.CLI.main(["issue", "list"])') before fixing, confirmed the fix resolves it after
• New test (missing_api_key_test.exs, async: false mirroring LinearCli.ApiTest's existing env-var-unset pattern) - clear message, exit 78, no raw Ash dump
• mix test - no regressions (14 pre-existing GPG/pinentry sandbox failures reproduce identically on a clean stash of this branch, unrelated)
• mix format --check-formatted clean

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Improves the CLI’s UX when LINEAR_API_KEY is missing by intercepting the specific Ash-wrapped “missing api key” error shape and emitting a clear, actionable configuration message while exiting with EX_CONFIG (78), instead of falling through to the generic “What the heck is this?” handler.

Changes:

• Add a dedicated handle_error/3 clause for the Ash %Ash.Error.Unknown{...} shape produced when LinearCli.Api.call/2 returns {:error, :missing_api_key}.
• Print a targeted message (with a settings URL) and exit 78 for missing configuration.
• Add an async: false regression test that unsets/restores LINEAR_API_KEY and asserts on stderr output + exit code.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
app/lib/linear_cli/cli.ex	Adds a specific error handler for the “missing API key” Ash error wrapper and exits with 78 + friendly guidance.
app/test/linear_cli/cli/missing_api_key_test.exs	Adds a non-async test ensuring the CLI prints the friendly message and exits 78 when LINEAR_API_KEY is unset.

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