Add community infrastructure: issue + PR templates, CoC

Gitea issue templates (.gitea/issue_template/):
- bug.yml: structured form requiring version, environment,
  what-happened, repro steps, expected, optional logs. Routes
  security reports to security@librete.ch instead of public
  issues.
- feature.yml: prompts for the underlying problem before the
  proposed solution, plus alternatives and out-of-scope.

Pull request template (.gitea/PULL_REQUEST_TEMPLATE.md):
checklist for tests, lint, manual exercise, docs, and changelog.
Asks for explicit reviewer notes so trade-offs surface in the
PR description rather than being lost in chat.

CODE_OF_CONDUCT.md: links to Contributor Covenant 2.1 verbatim
rather than inlining; documents scope, reporting address
(conduct@librete.ch), and points enforcement at the Covenant's
own Enforcement Guidelines.

README links the docs/ tree, CONTRIBUTING, and the CoC so new
contributors find the entry points.

Closes #31.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/issue_template/feature.yml

@@ -0,0 +1,38 @@
+name: Feature request
+about: Suggest a new capability or improvement
+labels: ["enhancement"]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Thanks for proposing a feature. Before opening a request,
+        please skim the open issues to see if it already exists.
+  - type: textarea
+    id: problem
+    attributes:
+      label: What problem does this solve?
+      description: |
+        Describe the user need first, not the implementation.
+        "I can't find old notes when I have hundreds" is more
+        useful than "Add full-text search."
+    validations:
+      required: true
+  - type: textarea
+    id: proposal
+    attributes:
+      label: Proposed solution
+      description: How would you like it to work?
+    validations:
+      required: true
+  - type: textarea
+    id: alternatives
+    attributes:
+      label: Alternatives considered
+      description: What other approaches did you think about, and why are they worse?
+  - type: textarea
+    id: scope
+    attributes:
+      label: Scope
+      description: |
+        Out of scope is just as useful as in scope. What does
+        this proposal NOT include?