fix(opencode): evict gitignored files from shadow snapshot index

[autopilotgrowth]

Refs #20695 (memory megathread — the "files staged before being added to .gitignore stay forever" case described by @thdxr).

Problem

add() only ran check-ignore against files that were in the current source candidate set (diff-files + ls-files --others --exclude-standard). Once a file was staged in the shadow repo on an early turn, then added to .gitignore but not modified afterwards, it fell out of both candidate lists — so the existing ignore() / drop() path never saw it.

It stayed in the shadow index forever, got rehashed into every write-tree, and bloated memory when sessions with large ex-untracked directories (node_modules, .android-sdk, .venv, …) were resumed.

Fix

Also list the shadow repo's currently-cached paths with git ls-files --cached, union them into the ignore() check, and let the existing drop() call evict anything that now matches. One extra git call per track(), negligible for sane repos, and it finally lets the user recover memory by just adding the offender to .gitignore without needing to wipe ~/.local/share/opencode/snapshot.

Verification

New test "untracked files retroactively gitignored are evicted from shadow index" reproduces the bug: cache/big.bin and cache/other.bin get staged in the first track(), cache/ is added to .gitignore, the files are not touched, and the second track() ought to drop them. I confirmed the test fails against dev and passes with this change. It inspects the shadow tree directly via git ls-tree -r because diffFull (correctly) filters gitignored paths from the user-facing diff, hiding the very rows we need to assert on.

Full snapshot suite still passes (52 pass, 1 skip, 1 pre-existing chmod-not-found failure on Windows that also fails on clean dev).

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