fix: treat watchdog as always-on hard dependency (#31)

[microsasa]

Closes #31

Problem

watchdog>=4 is listed as a hard production dependency in pyproject.toml, but two places treat it as optional:

1. Dead except ImportError guard in _start_observer — unreachable because watchdog is always installed
2. Misleading README — tells users to run uv add watchdog for a feature that's always active
3. _stop_observer getattr dance — unnecessary indirection since the watchdog API is always present

Solution (Option B — always-on)

• cli.py: Import Observer at module top level; remove try/except ImportError in _start_observer; simplify _stop_observer to call .stop() / .join() directly
• README.md: Replace conditional phrasing with "The display auto-refreshes when session files change (2-second debounce)."
• implementation.md / plan.md: Remove "if watchdog is installed" language from docs

Tests

• Added test_start_observer_returns_running_observer — asserts _start_observer returns a non-None, alive observer for an existing directory, then cleans up via _stop_observer

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[Copilot]

Pull request overview

This PR aligns the codebase and documentation with the fact that watchdog>=4 is a hard production dependency, removing now-dead “optional dependency” handling and clarifying auto-refresh behavior.

Changes:

• Simplifies watchdog observer lifecycle in the CLI by importing Observer at module scope and removing dead ImportError/getattr indirection.
• Updates README and internal docs to describe auto-refresh as always-on (with the existing 2s debounce).
• Adds a unit test to assert _start_observer returns a running observer and cleans it up via _stop_observer.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/copilot_usage/cli.py	Makes watchdog observer usage unconditional and simplifies start/stop logic.
tests/copilot_usage/test_cli.py	Adds coverage for starting/stopping the watchdog observer.
README.md	Removes misleading optional-install instructions for watchdog.
src/copilot_usage/docs/plan.md	Removes “if watchdog is installed” phrasing from the plan.
src/copilot_usage/docs/implementation.md	Updates the observer description to match always-on behavior and existing directory check.

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