PR Review: Consolidate multiple tools into unified gomailtest binary

Overall this is a clean, well-structured change. The build workflow simplification is logical and the documentation updates are consistent throughout — good work. I found a few issues worth addressing before merge.

Bug: Stale "Tools Overview" table in release notes (line 233–239)

This is the main actionable issue. The table at the bottom of the release body was not updated and still references the old standalone tool names:

| Tool | Protocol | Ports | Auth Methods |
|------|----------|-------|--------------|
| smtptool | SMTP | 25, 587, 465 | PLAIN, LOGIN, CRAM-MD5, XOAUTH2 |
| imaptool | IMAP | 143, 993 | PLAIN, LOGIN, XOAUTH2 |
| pop3tool | POP3 | 110, 995 | USER/PASS, APOP, XOAUTH2 |
| jmaptool | JMAP | 443 | Basic, Bearer |
| msgraphtool | Graph API | 443 | Client Secret, Certificate, Bearer |

Every GitHub Release created after this merges will show these tool names even though they no longer exist as standalone binaries. Suggested replacement:

| Subcommand | Protocol | Ports | Auth Methods |
|------------|----------|-------|--------------|
| gomailtest smtp | SMTP | 25, 587, 465 | PLAIN, LOGIN, CRAM-MD5, XOAUTH2 |
| gomailtest imap | IMAP | 143, 993 | PLAIN, LOGIN, XOAUTH2 |
| gomailtest pop3 | POP3 | 110, 995 | USER/PASS, APOP, XOAUTH2 |
| gomailtest jmap | JMAP | 443 | Basic, Bearer |
| gomailtest msgraph | Graph API | 443 | Client Secret, Certificate, Bearer |

Minor: Flag style inconsistency in Quick Start examples

The SMTP/IMAP/POP3/JMAP examples use --help (double-dash, typical for subcommand CLIs), but the Microsoft Graph example still uses single-dash flags:

# Other subcommands (double-dash style via --help)
./gomailtest smtp testconnect -host smtp.example.com -port 25

# msgraph still uses single-dash flags
./gomailtest msgraph -tenantid "..." -clientid "..." -secret "..." \
  -mailbox "user@example.com" -action getinbox

If msgraph genuinely uses a different flag style internally, a brief comment noting that would help users. If not, the flags should be consistent across the Quick Start section.

Nit: GOOS not explicitly set in the build step

The build step sets GOARCH but not GOOS:

- name: Build binary
  env:
    GOARCH: ${{ matrix.goarch }}
  run: |
    go build -ldflags="-s -w" -o bin/gomailtest${{ matrix.ext }} ./cmd/gomailtest

This works correctly today because each runner builds for its own host OS. However, if the matrix is ever extended to support cross-compilation, the missing GOOS will cause silent issues. Consider adding it explicitly (e.g., GOOS: linux / GOOS: windows / GOOS: darwin) alongside GOARCH in the matrix include entries.

Observation: Archive vs binary name

The ZIP files remain named gomailtesttool-* while the binary inside is gomailtest. This is a reasonable migration choice (existing download links stay valid), but it's worth a comment in the README or release notes to avoid confusion for users who see the archive name and expect to find a gomailtesttool binary inside.

Summary: The stale "Tools Overview" table is the only change I'd consider required before merge — it will surface in every release. The rest are minor polish items.