fix(fs): stop truncating names with spaces in companion ls/find output

[mornyx]

Fixes #73

Problem

ti fs list-files and ti fs find-files returned alpha for a file named alpha beta.txt (reproduced across three directories; ti fs grep-file-content shares the path parser and is equally affected). The service data is intact — the HTTP API returns the full name — this is purely companion-output parsing:

• parseDrive9LS split drive9 fs ls -l lines with strings.Fields and kept fields[2], but the companion prints kind<TAB>size<TAB>name through a space-padding tabwriter — the name is the final column and may contain spaces, so it was cut at the first space.
• parseDrive9Paths kept fields[0] of each line, but drive9 fs find prints the whole path per line (and fs grep prints path<TAB>score), so paths containing spaces were truncated at the first space.

Change

• parseDrive9LS now parses the two fixed tokens (kind, size) and keeps the remainder verbatim as the name; internal (including multiple consecutive) spaces survive. Malformed lines (non-numeric size, missing name) are skipped instead of yielding size-0 entries.
• parseDrive9Paths treats each line as a path and cuts only at a TAB (grep's score separator), preserving spaces in paths; the : prefix strip is unchanged.

Known limitation: a filename that starts with spaces still loses them in ls -l, because the tabwriter's column padding is indistinguishable from them in the text format.

Test

• TestParseDrive9LSPreservesNamesWithSpaces
• TestParseDrive9LSSkipsMalformedLines
• TestParseDrive9PathsPreservesSpaces (find paths, :-prefixed paths, grep path<TAB>score, limit)

Full go test ./... passes.

[qiffang]

Review round 1 — exact HEAD 204a3261be8c120d2209d6cb48b42d9b28646b95. Verdict: APPROVED.

本轮修复与真实 companion 输出合同一致，未发现 blocker：

• drive9 fs ls -l 的 producer 使用 tabwriter 输出 kind<TAB>size<TAB>name；实际展开后只有 kind/size 是固定 token，name 是最终列。新 parser 只解析前两个 token、保留 name 中的内部及连续空格，并对缺 name/非法 size 的行 fail-closed 跳过，不再伪造 size=0 entry。
• drive9 fs find 每行输出完整 path；fs grep 仅在 score 存在时输出 path<TAB>score。parseDrive9Paths 只在 TAB 处截断，因而保留 path 内部空格；既有 : remote-prefix、basename 和 limit 语义保持。
• 我用 Drive9 producer 的相同 text/tabwriter 配置复现了实际 ls 排版，结果与新 fixture 一致。将生产 parser 临时恢复到 base 旧实现后，3 个新增回归（ls 空格、malformed 行、find/grep 空格）均确定性失败，证明测试对原 bug 敏感。

验证：目标测试 -count=20、go vet ./internal/fs、全仓 build、diff check 均通过；exact HEAD 为 MERGEABLE，GitHub test 与 CLA checks 均 SUCCESS。无遗留 blocker。