host: improve CLI help text accessibility across all utilities

[Copilot]

The --help output for all three host utilities was bare option lists with no descriptions,<pr_title>
host: improve CLI help text for better accessibility
</pr_title>

<pr_description>
The --help output across all three host utilities was bare option lists with no descriptions, defaults, or usage context — unhelpful to new users and easy to misuse.

Changes

• All three tools (simtrace2-sniff, simtrace2-cardem-pcsc, simtrace2-tool): added Usage: line and Options: header; added plain-English description for every option
• simtrace2-sniff / simtrace2-cardem-pcsc: noted hex format and default values for -i/-V/-P; added (required) markers on mandatory vendor/product ID flags
• simtrace2-cardem-pcsc: clarified -Z --set-sim-presence polarity (0=active-low, 1=active-high); added default for -n --pcsc-reader-num
• simtrace2-tool: added per-command descriptions; added Examples: section

Before/after for simtrace2-tool:

# Before
Commands:
    modem reset (enable|disable|cycle)
    modem sim-switch (local|remote)
    modem sim-card (insert|remove)

# After
Commands:
    modem reset (<pr_title>
host: improve CLI help text for better accessibility
</pr_title>

<pr_description>
The `--help` output across all three host utilities was bare option lists with no descriptions, defaults, or usage guidance — unhelpful to first-time users.

## Changes

- **All three tools**: Added `Usage:` line and `Options:` section header to `print_help()`
- **Per-option descriptions**: Every flag now has a plain-English description inline, e.g.:

-k --keep-running Restart automatically when the USB device disconnects
-i --gsmtap-ip A.B.C.D IP address of GSMTAP receiver for Wireshark (default: 127.0.0.1)

- **`simtrace2-cardem-pcsc`**: Marked `-V`/`-P` as required; clarified `-Z` polarity (`0=active-low, 1=active-high`); added default for `-n`
- **`simtrace2-tool`**: Added per-command descriptions and an `Examples:` section:

simtrace2-tool -V 1d50 -P 4001 modem reset cycle
simtrace2-tool -V 1d50 -P 4001 modem sim-switch remote
simtrace2-tool -V 1d50 -P 4001 modem sim-card insert

No functional logic changed.

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<details>

<summary>Original prompt</summary>

> yes


</details>