fix(opencode): reduce memory allocations in read tool

[SeanThomasWilliams]

Fixes #13669

Reduce transient memory allocations in ReadTool:

• Bounded file.slice(0, estimate).text() instead of file.text() for text reads (issue item 1)
• Binary detection reads only first 4KB via file.slice(0, 4096).arrayBuffer() (issue item 2)
• Async readdir for missing-file suggestions

Item 3 from the issue (image/PDF base64 size guard) is not addressed here — capping inline images would be a feature regression since all sizes currently work. That can be revisited separately if needed.

Tests

$ cd packages/opencode && bun test test/tool/read.test.ts --timeout 30000

bun test v1.2.20 (6ad208bc)

 33 pass
 0 fail
 59 expect() calls
Ran 33 tests across 1 file. [2.06s]

Memory verification

100MB text file read, peak RSS measured by polling the child process VmRSS.
The opencode launcher spawns a native binary child, so pgrep -P finds the actual process to sample.

STOCK=$(npm root -g)/opencode-ai/node_modules/opencode-linux-x64/bin/opencode
PATCHED=packages/opencode/dist/opencode-linux-x64/bin/opencode

# Generate 100MB fixture
(yes "line lorem ipsum dolor sit amet" || true) 2>/dev/null \
  | head -c 104857600 > /tmp/large.txt

# Measure one run (repeat for each binary)
measure() {
  OPENCODE_BIN_PATH="$1" opencode run \
    "Use the read tool on /tmp/large.txt with offset 1 and limit 2000. Reply OK." \
    >/dev/null 2>/dev/null &
  local parent=$!
  local child="" peak=0

  # Wait for native binary child to spawn
  for i in $(seq 1 40); do
    child=$(pgrep -P $parent 2>/dev/null | head -1)
    [ -n "$child" ] && break
    sleep 0.25
  done

  # Poll VmRSS every 50ms
  while kill -0 $parent 2>/dev/null; do
    local rss=$(awk '/^VmRSS/{print $2}' /proc/${child}/status 2>/dev/null)
    [ "${rss:-0}" -gt "$peak" ] && peak=$rss
    sleep 0.05
  done
  wait $parent 2>/dev/null
  echo "$peak"
}

echo "stock:   $(measure $STOCK) KB"
echo "patched: $(measure $PATCHED) KB"
rm /tmp/large.txt

Results (3 runs each):

label      run  peak_rss_kb
stock        1       590580
stock        2       602804
stock        3       600364
patched      1       394936
patched      2       387172
patched      3       386440

stock   mean=584 MB  stddev=6.3 MB
patched mean=380 MB  stddev=4.6 MB
delta   -35% peak RSS

Small file control (100 lines) — no regression:

stock-small   peak_rss = 356 MB
patched-small peak_rss = 378 MB  (within noise)

Edge cases verified

Case	Result
Empty file (0 bytes)	Reads successfully, reports 1 blank line
Single 5MB line (no newlines)	Returns truncated first line
Offset past EOF (offset=9999 on 2-line file)	Error: Offset 9999 is out of range
Binary file (random bytes)	Error: Cannot read binary file
Unicode/multibyte (CJK, emoji, accents)	All characters preserved correctly

[github-actions]

Hey! Your PR title perf(tool): reduce memory allocations in ReadTool doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[SeanThomasWilliams]

Superseded by #14009 and #14027 which landed on dev with equivalent fixes (streaming text reads, bounded binary detection, async readdir).