feat: add zemo dump to print all memos

[hidetzu]

Summary

Add a zemo dump subcommand that prints scratch and every topic to stdout under === <name> === section headers, ordered as scratch first then topics alphabetically. Read-only — no git operations. Completes the search/discovery primitive set (ls / cat / dump) so terminal-side search such as WezTerm QuickSearch can grep across all memos in a single pass.

This PR also bumps the version to 0.2.0, intended to be tagged immediately after merge.

Related issue

Closes #5

Changes

• New Command.dump variant in cli.zig; routed from parseArgs (rejects extra args) and dispatched in run.
• New doDump (env → memo dir) and dumpMemos (path-based, testable) helpers.
• Section header format: === scratch === for scratch, === <topic> === for each topic. Blank lines before/after the header for readability; previous-section trailing-newline tracking avoids accidental double blanks between sections.
• Targets: <memo>/scratch.txt (printed first if present) followed by alphabetically sorted <memo>/topics/*.md. Subdirectories and non-.md files under topics/ are excluded. Missing scratch / missing topics/ are skipped silently (exit 0 with no output if both are missing).
• File contents streamed via Io.Reader.streamRemaining (4 KiB read buffer) — bounded memory regardless of file size.
• Refactored: listTopics was split — directory iteration / filter / sort moved into a shared collectTopicNames helper that both listTopics and dumpMemos call. errdefer (not defer) is used in the helper so the returned list is not freed on success.
• README: added zemo dump to Features and Usage sections.
• HELP_TEXT: added zemo dump line.
• Version bump: VERSION constant in cli.zig updated from 0.1.0 to 0.2.0. After merge, tag v0.2.0 to trigger the release workflow (binaries + auto-generated release notes covering feat: add zemo ls to list topics #6 ls, feat: add zemo cat to print memo to stdout #7 cat, feat: add zemo dump to print all memos #8 dump, plus the relative-ZEMO_DIR fix from feat: add zemo ls to list topics #6).

Testing

• zig build test passes locally
• Manually verified the affected command(s) (zig build run -- dump against ~/memo, zig build run -- --version reports zemo 0.2.0)
• Cross-build check (if touching platform-specific code): zig build -Dtarget=x86_64-windows / -Dtarget=x86_64-macos

New tests added:

• parseArgs: dump subcommand — parser-level
• dumpMemos: scratch only — only scratch present
• dumpMemos: topics only sorted and filtered — alphabetical sort, .md filter, subdirectory exclusion
• dumpMemos: scratch and topics — full combined output, including blank-line spacing across sections with/without trailing newlines
• dumpMemos: neither scratch nor topics prints nothing — empty memo case
• doDump: uses ZEMO_DIR override — end-to-end through env-map resolution

Notes for reviewers

• The blank-line handling in writeSectionHeader (previous_ended_with_newline + wrote_any flags) is what enables the spec'd === a ===\n\n<a>\n\n=== b ===\n\n<b> shape regardless of whether each file ends with a trailing newline. Tests cover both shapes.
• Extracting collectTopicNames was a deliberate scope addition to avoid duplicating the iterate-filter-sort logic between listTopics and dumpMemos. The change to listTopics is mechanical (delegates to the helper) and remains covered by its existing test.
• Test helpers writeTestFile / tmpDirPath are new — small wrappers used by the four dumpMemos tests to keep each test focused on the assertions rather than tmp-dir bookkeeping.
• Version bump is included in this PR so that "merge → tag" is a single step. After merge: git checkout main && git pull && git tag v0.2.0 && git push origin v0.2.0.