feat: progress bars for CLI walk stages

[codeninja]

Summary

wikifi walk runs through several stages with no visible progress feedback. On a real codebase the walk can take a while, and right now it's opaque — the user can't tell whether it's working, stalled, or how far along it is.

Add progress bars / status indicators surfacing the walk's four stages:

1. Introspect — repository root structure review and scope decision
2. Filter — dropping unstructured / near-empty input before extraction
3. Extract — per-file deterministic extraction (good candidate for a per-file count progress bar)
4. Synthesize — section synthesis from the aggregate, including derivative sections

Scope

• Per-stage indication of which stage is active
• For the Extract stage, a file-count progress bar (n / total files)
• Respect non-TTY / piped output (degrade to plain log lines, no ANSI spinner)
• Keep it inside the CLI presentation layer — no LLM agency or behavior change in the walk itself

Acceptance criteria

• Running wikifi walk shows the active stage and extract-phase file progress
• Output is clean when stdout is not a TTY
• Tests cover the progress reporting (≥85% coverage maintained)