feat: add `zemo search` command

[hidetzu]

Motivation

Closes the discoverability loop for the read-only command set (ls #3, cat #4, dump #5). dump works for cross-memo search via the terminal's own search (e.g. WezTerm QuickSearch), but a dedicated search command is more efficient for the common "find lines that mention X" case and produces output (path:line:match) that pipes naturally into pickers like fzf.

Spec

zemo search <query> — print lines matching <query> across scratch and all topic files.

• Targets:
  • <memo>/scratch.txt
  • <memo>/topics/*.md
  • Skip subdirectories and non-.md files under topics/, matching dump (feat: add zemo dump command #5) and ls (feat: add zemo ls command #3).
• Match semantics: fixed-string, case-sensitive substring match. (Regex / case-insensitive can be added later.)
• Output format: <relative-path>:<line-number>:<line-content> per match, one per line. Path is relative to <memo> (e.g. topics/git.md:42:see also git rebase). Matches git grep -n style so output pipes into fzf, bat, etc.
• Ordering: scratch first, then topics alphabetical (matching dump). Within a file, order by line number ascending.
• Missing files: scratch missing → skip silently. topics/ missing → skip silently. Both missing → no output, exit 0.
• No matches: no output, exit 1 (standard grep-like behaviour).
• Empty query: usage error, exit 2.
• Read-only: no git operations.

Example

$ zemo search rebase
scratch.txt:3:remember to git rebase before push
topics/git.md:12:`git rebase --interactive` to squash commits
topics/git.md:34:rebase vs merge — use rebase for feature branches

Implementation note

Implement as a Zig file walk over <memo>/scratch.txt and <memo>/topics/*.md, line-by-line fixed-string match. No git dependency, consistent with cat (#4), ls (#3), dump (#5). Memo repos are personal-scale (hundreds to low thousands of small markdown files), so a straightforward Zig walk is expected to be fast enough indefinitely.

Scope

In:

• Resolve <memo> via ZEMO_DIR → default.
• Walk scratch.txt + topics/*.md, fixed-string match, write path:line:content to stdout.
• No-match → exit 1. Empty query → exit 2.

Out (separate issues):

• --regex / -E
• --ignore-case / -i
• --files-only / -l
• --topic <topic> (scope to one file)
• --context <n> / -C
• Color output

Acceptance criteria

• zemo search <query> prints all matching lines across scratch.txt and topics/*.md in path:line:content format.
• Output ordering: scratch first, then topics alphabetical, then by line number ascending.
• Non-.md files and subdirectories under topics/ are excluded.
• No matches → no output, exit 1.
• Missing scratch / missing topics dir → skip silently.
• Empty query → usage error, exit 2.
• No git operations are performed.
• Tests cover: matches in scratch only, matches in topics only, mixed, no matches, missing files, empty query.
• zig build test passes.
• README gains an entry for zemo search and an fzf integration recipe.