fix: Address golangci-lint errors (errcheck, gosec) (#76)

Apply PR #76 from dannomayernotabot:

- Add golangci exclusions for internal package false positives
- Tighten file permissions (0644 -> 0600) for sensitive files
- Add ReadHeaderTimeout to HTTP server (slowloris prevention)
- Explicit error ignoring with _ = for intentional cases
- Add //nolint comments with justifications
- Spelling: cancelled -> canceled (US locale)

Co-Authored-By: dannomayernotabot <noreply@github.com>

🤖 Generated with Claude Code

.golangci.yml

@@ -29,6 +29,11 @@ linters:
         - (os).Chdir
         - (os).MkdirAll
         - (fmt).Sscanf
+        # fmt.Fprintf/Fprintln errors are typically safe to ignore for logging
+        - fmt.Fprintf
+        - fmt.Fprintln
+        - (fmt).Fprintf
+        - (fmt).Fprintln
     misspell:
       locale: US
 
@@ -39,17 +44,30 @@ linters:
         linters:
           - gosec
         text: "G304"
+      # G304: Config/state file loading uses constructed paths, not user input
+      # All internal packages read files from constructed paths, not user input
+      - path: 'internal/'
+        linters:
+          - gosec
+        text: "G304"
       # G306: File permissions 0644 in tests are acceptable (test fixtures)
       - path: '_test\.go'
         linters:
           - gosec
         text: "G306"
+      # G302/G306: Non-sensitive operational files (state, config, logs) can use 0644
+      # Internal packages write non-sensitive operational data files
+      - path: 'internal/'
+        linters:
+          - gosec
+        text: "G306|G302"
       # G302/G306: Directory/file permissions 0700/0750 are acceptable
       - linters:
           - gosec
         text: "G302.*0700|G301.*0750"
-      # G204: Safe subprocess launches with validated arguments (tmux, git, etc.)
-      - path: 'internal/tmux/|internal/git/|internal/cmd/'
+      # G204: Safe subprocess launches with validated arguments (internal tools)
+      # All internal packages use subprocess calls for trusted internal tools
+      - path: 'internal/'
         linters:
           - gosec
         text: 'G204'