feat(release): changelog fragments directory + build script (issue #222)

CHANGELOG.d/_template.md

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+<!-- one file per PR, named <PR>-<slug>.md -->
+<!-- PR is the GitHub PR number (e.g. 222, 235). If you don't have one yet, -->
+<!-- use the next free 4-digit slot and rename on PR creation.               -->
+<!--
+  Slug is a short kebab-case label (e.g. changelog-d, subagent-cli).
+
+  The aggregator sorts fragments by PR number ascending, so the order
+  in CHANGELOG.md is deterministic and conflict-free across PRs.
+
+  Two sections are recognised per fragment:
+
+    ### Added — <feature name> (PR #<n>)
+    ### Changed — <what changed> (PR #<n>)
+    ### Fixed — <what was broken> (PR #<n>)
+    ### Removed — <what is gone> (PR #<n>)
+
+  Each section is one line: "<category> — <headline> (PR #<n>)".
+  Body lines starting with `-` follow on subsequent lines.
+
+  Example:
+
+    ### Added — Workspace checkpointing + rewind (PR #235)
+
+    - Content-addressed snapshot store backed by modernc.org/sqlite
+    - `sin-code checkpoint [label]` captures the dirty set
+    - `sin-code rewind [id|--list]` restores the workspace
+    - `Loop.BeforeMutate` auto-snapshots `sin_write`/`sin_edit`
+-->