feat: add partial context matching to switch command

[davidblum]

Overview

When running switch <name> with a partial context name, the tool currently requires an exact match or returns an error. This PR adds partial matching support so users can type a substring and select from matching contexts.

Changes

Behavior:

• Exact match: switches immediately (preserves existing behavior)
• Partial match: opens fuzzy finder pre-filtered to matching contexts
• No match: returns error no contexts matching "<name>"

Matching is case-insensitive for partial matches and searches both context names and aliases.

Files changed:

• pkg/main.go: Added waitForSearchResults() helper and filtering logic in Switcher()
• cmd/switcher/switcher.go: Unified code path to pass optional desiredContext to Switcher()

Example

# Before: exact match required
switch prod-us-east-1    # works
switch prod              # error: context not found

# After: partial matching
switch prod              # opens picker with contexts containing "prod"
switch prod-us-east-1    # still works (exact match)