fix(timeout): accept Go duration strings and report timeouts clearly (#12)

[AzarAI-TOP]

Description

Fixes #12.

The --timeout / -t flag parsed its value with time.ParseDuration(value + "s"), unconditionally appending s. That only worked for bare-second numbers and mishandled the duration strings the issue explicitly asks to support:

Input	Old behaviour	New behaviour
--timeout 30	30s ✓	30s ✓ (unchanged)
--timeout 1m	1ms — 1 millisecond, silently wrong	1m ✓
--timeout 5s	error (5ss)	5s ✓
--timeout 500ms	error (500mss)	500ms ✓
--timeout notaduration	confusing parse error	invalid timeout "notaduration": want a number of seconds or a duration like 5s, 1m, 500ms

Changes

• parseTimeout helper (main.go): a bare number is treated as seconds (keeps the documented --timeout <seconds> default), anything else is parsed as a Go duration string (5s, 1m, 500ms, 1m30s). Negative / unparseable values now error clearly.
• Clear timeout message (client/client.go): when a request actually times out, surface request timed out after 30s (increase or disable with --timeout) instead of the raw context deadline exceeded transport error.
• Updated the --timeout help text.
• No change to the default (30s) or to the saved-request (run) path.

Maps to the issue's acceptance criteria:

• --timeout flag accepts duration strings
• Applied to the HTTP client
• Clear error message on timeout (not a generic Go error)
• Default remains a reasonable 30s
• Test coverage added

Type of change

• Bug fix (non-breaking change which fixes an issue)
• This change requires a documentation update (help text updated in-tree)

How Has This Been Tested?

• Added unit tests — CLI parser (main_test.go, incl. a regression test that 1m == 1 minute, not 1ms) and client timeout behaviour (client/timeout_test.go, a firing timeout + a request that completes in time).
• Tested via go test ./... locally — all packages pass.
• Compiled and tested via local binary:
  • kurl --timeout 500ms … → works
  • kurl --timeout 1ms https://example.com → request timed out after 1ms (increase or disable with --timeout)
  • kurl --timeout notaduration … → clear validation error

Checklist

• My code follows the style guidelines of this project (gofmt, go vet clean)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (help text)
• My changes generate no new warnings
• I have added tests that prove my fix is effective
• New and existing unit tests pass locally with my changes