Add ExtensionContext.paths to expose resolved TauPaths

[AmirF194]

ExtensionRuntime.load() receives a TauResourcePaths that carries the resolved TauPaths, but never stores it. ExtensionContext exposes cwd, model, session_id, and other session values, but nothing about the Tau home, so an extension has no way to find TauPaths.home when a host moved it with TauPaths(home=...). Extensions end up hardcoding ~/.tau, which reads and writes the wrong location once the home moves (the tau-subagents extension does this in four places, recorded as a workaround in its ADR 0003).

Fix: ExtensionRuntime.load() now stores paths.paths or TauPaths(home=paths.root), the same fallback session.py already uses to build the trust store, and exposes it through a new ExtensionRuntime.paths property. ExtensionContext.paths reads that property under the same assert_active() generation check as every other context property, so a context captured before a /reload raises ExtensionError instead of reading the reloaded world.

Verification:

• New test test_context_paths_exposes_resolved_paths_from_load and test_context_paths_falls_back_to_resource_root_home fail on main with AttributeError: 'ExtensionContext' object has no attribute 'paths' and pass on this branch (both run in the same Docker image).
• test_session_exposes_extension_paths_at_moved_home exercises the real CodingSession.load() path end to end, same fail-before/pass-after check.
• Added a context.paths staleness assertion next to the existing context.cwd one in test_reset_for_reload_invalidates_prior_context_and_ui.
• Full suite (uv run pytest), ruff check ., ruff format --check ., and mypy all pass.

Fixes #598

[AmirF194]

No rush, just checking in on this one. CI is sitting behind the first-time-contributor workflow approval wall, so it has not actually run yet, happy to rebase again if that helps.