Add progress indicator after detecting staged changes

[Iron-Ham]

Summary

• Adds an "Analyzing staged changes…" progress message after detecting staged changes to provide user feedback during potentially long-running operations
• Fixes a pre-existing failing test that was checking for outdated prompt text

Fixes #19

Stack

This PR is part of a stack addressing #14 (stalling after detecting staged changes):

1. PR Enforce LLM timeout configuration that was previously dead code #20 - Fix LLM timeout enforcement (Fixes LLM timeout configuration is defined but never enforced #16)
2. PR Add timeout and fix resource leaks in Git client #21 - Fix Git timeout + resource leaks (Fixes Git process operations have no timeout and leak file descriptors #17)
3. PR Add CLI and config file support for LLM timeout settings #22 - Add timeout configuration CLI/config (Fixes Add CLI flag and config file support for LLM timeout settings #18)
4. PR Add progress indicator after detecting staged changes #15 (this PR) - Progress logging (Fixes Add progress logging during diff analysis #19)

Details

The tool appeared to "stall" after printing [NOTICE] Detected N staged change(s). because there was no progress feedback while potentially long-running operations executed:

1. git diff --cached (can be slow for large repos or with --function-context)
2. Diff parsing and summarization
3. LLM inference

Before:

[INFO] Repository root: /path/to/repo
[NOTICE] Detected 11 staged change(s).
<long silence>
[NOTICE] Summary: 11 file(s), +234 / -89

After:

[INFO] Repository root: /path/to/repo
[NOTICE] Detected 11 staged change(s).
[INFO] Analyzing staged changes…
[NOTICE] Summary: 11 file(s), +234 / -89

Test plan

• Build passes (swift build)
• All 49 tests pass (swift test)
• Verified CLI works with scg generate --help