feat: migrate from docopt to cobra for CLI argument parsing

Replace docopt-go with spf13/cobra, giving cheat a built-in
`--completion` flag that dynamically generates shell completions for
bash, zsh, fish, and powershell. This replaces the manually-maintained
static completion scripts and resolves the root cause of multiple
completion issues (#768, #705, #633, #632, #476).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

adr/002-environment-variable-parsing.md

@@ -8,7 +8,7 @@ Accepted
 
 ## Context
 
-In `cmd/cheat/main.go` lines 47-52, the code parses environment variables assuming they all contain an equals sign:
+In the `envVars()` function in `cmd/cheat/main.go`, the code parses environment variables assuming they all contain an equals sign:
 
 ```go
 for _, e := range os.Environ() {

adr/003-search-parallelization.md

@@ -100,5 +100,5 @@ The parallelization attempt was valuable as a learning exercise and definitively
 
 ## References
 
-- Benchmark implementation: cmd/cheat/search_bench_test.go
+- Benchmark implementation: test/integration/search_bench_test.go
 - Reverted parallel implementation: see git history (commit 82eb918)