chore: bump version to 4.5.0

Bug fixes:
- Fix inverted pager detection logic (returned error instead of path)
- Fix repo.Clone ignoring destination directory parameter
- Fix sheet loading using append on pre-sized slices
- Clean up partial files on copy failure
- Trim whitespace from editor config

Security:
- Add path traversal protection for cheatsheet names

Performance:
- Move regex compilation outside search loop
- Replace string concatenation with strings.Join in search

Build:
- Remove go:generate; embed config and usage as string literals
- Parallelize release builds
- Add fuzz testing infrastructure

Testing:
- Improve test coverage from 38.9% to 50.2%
- Add fuzz tests for search, filter, tags, and validation

Documentation:
- Fix inaccurate code examples in HACKING.md
- Add missing --conf and --all options to man page
- Add ADRs for path traversal, env parsing, and search parallelization
- Update CONTRIBUTING.md to reflect project policy

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

doc/adr/002-environment-variable-parsing.md

@@ -0,0 +1,100 @@
+# ADR-002: No Defensive Checks for Environment Variable Parsing
+
+Date: 2025-01-21
+
+## Status
+
+Accepted
+
+## Context
+
+In `cmd/cheat/main.go` lines 47-52, the code parses environment variables assuming they all contain an equals sign:
+
+```go
+for _, e := range os.Environ() {
+    pair := strings.SplitN(e, "=", 2)
+    if runtime.GOOS == "windows" {
+        pair[0] = strings.ToUpper(pair[0])
+    }
+    envvars[pair[0]] = pair[1]  // Could panic if pair has < 2 elements
+}
+```
+
+If `os.Environ()` returned a string without an equals sign, `strings.SplitN` would return a slice with only one element, causing a panic when accessing `pair[1]`.
+
+## Decision
+
+We will **not** add defensive checks for this condition. The current code that assumes all environment strings contain "=" will remain unchanged.
+
+## Rationale
+
+### Go Runtime Guarantees
+
+Go's official documentation guarantees that `os.Environ()` returns environment variables in the form "key=value". This is a documented contract of the Go runtime that has been stable since Go 1.0.
+
+### Empirical Evidence
+
+Testing across platforms confirms:
+- All environment variables returned by `os.Environ()` contain at least one "="
+- Empty environment variables appear as "KEY=" (with an empty value)
+- Even Windows special variables like "=C:=C:\path" maintain the format
+
+### Cost-Benefit Analysis
+
+Adding defensive code would:
+- **Cost**: Add complexity and cognitive overhead
+- **Cost**: Suggest uncertainty about Go's documented behavior
+- **Cost**: Create dead code that can never execute under normal conditions
+- **Benefit**: Protect against a theoretical scenario that violates Go's guarantees
+
+The only scenarios where this could panic are:
+1. A bug in Go's runtime (extremely unlikely, would affect all Go programs)
+2. Corrupted OS-level environment (would cause broader system issues)
+3. Breaking change in future Go version (would break many programs, unlikely)
+
+## Consequences
+
+### Positive
+- Simpler, more readable code
+- Trust in platform guarantees reduces unnecessary defensive programming
+- No performance overhead from unnecessary checks
+
+### Negative
+- Theoretical panic if Go's guarantees are violated
+
+### Neutral
+- Follows Go community standards of trusting standard library contracts
+
+## Alternatives Considered
+
+### 1. Add Defensive Check
+```go
+if len(pair) < 2 {
+    continue  // or pair[1] = ""
+}
+```
+**Rejected**: Adds complexity for a condition that should never occur.
+
+### 2. Add Panic with Clear Message
+```go
+if len(pair) < 2 {
+    panic("os.Environ() contract violation: " + e)
+}
+```
+**Rejected**: Would crash the program for the same theoretical issue.
+
+### 3. Add Comment Documenting Assumption
+```go
+// os.Environ() guarantees "key=value" format, so pair[1] is safe
+envvars[pair[0]] = pair[1]
+```
+**Rejected**: While documentation is good, this particular guarantee is fundamental to Go.
+
+## Notes
+
+If Go ever changes this behavior (extremely unlikely as it would break compatibility), it would be caught immediately in testing as the program would panic on startup. This would be a clear signal to revisit this decision.
+
+## References
+
+- Go os.Environ() documentation: https://pkg.go.dev/os#Environ
+- Go os.Environ() source code and tests