feat: add zemo cat to print memo to stdout

[hidetzu]

Summary

Add a zemo cat subcommand that prints memo content to stdout. With no argument it prints <memo>/scratch.txt; with a topic argument it prints <memo>/topics/<topic>.md. Read-only — no git operations are performed. Builds the second primitive (after zemo ls) for the search/discovery flow that delegates filtering to terminal-side search such as WezTerm QuickSearch.

Related issue

Closes #4

Changes

• New Command.cat: ?[]const u8 variant in cli.zig; routed from parseArgs (handles zemo cat and zemo cat <topic>) and dispatched in run.
• New doCat (env → file path resolution) and printFile (path-based, testable) helpers.
• Topic name validation reuses paths.isValidTopic (rejects path-traversal, dots, spaces, multibyte) with a friendly stderr message and exit code 1.
• Missing scratch / topic file → zemo: no such file: <path> to stderr, exit 1 (standard cat-like behaviour).
• File contents are streamed via Io.Reader.streamRemaining so memory usage is bounded by a 4 KiB read buffer regardless of file size.
• README: added zemo cat to Features and Usage sections.
• HELP_TEXT: added zemo cat and zemo cat <topic> lines.

Testing

• zig build test passes locally
• Manually verified the affected command(s):
  • zig build run -- cat zemo prints ~/memo/topics/zemo.md
  • zig build run -- cat prints scratch
  • zig build run -- cat doesnotexist → stderr zemo: no such file: ... and exit 1
• Cross-build check (if touching platform-specific code): zig build -Dtarget=x86_64-windows / -Dtarget=x86_64-macos

New tests added:

• parseArgs: cat without topic / parseArgs: cat with topic / parseArgs: cat too many args — parser-level
• printFile: prints file contents to stdout — integration test using std.testing.tmpDir and Io.Writer.Allocating to capture output
• printFile: missing file → exit 1, stderr message — verifies error path and exit code

Notes for reviewers

• printFile was extracted from doCat to keep the testable surface path-based (mirrors the listTopics / doLs split from feat: add zemo ls to list topics #6).
• Topic-name validation lives in doCat rather than parseArgs, matching the existing pattern for zemo <topic> (openTopic). This keeps parseArgs payload-agnostic and consolidates user-facing validation messages in the dispatcher.
• The relative-ZEMO_DIR fix from feat: add zemo ls to list topics #6 (centralized in paths.memoDir) carries over for free — doCat calls paths.memoDir and benefits from the absolute-path normalization.