feat: Add hashline-based file read and edit tools

[gadenbuie]

Implements the hashline approach described in The Harness Problem — content-hash line annotations for reliable, validated file editing.

Summary

• Hashline annotations in btw_tool_files_read: Each line returned to the model is now prefixed with line_number:hash| (e.g. 2:f1a| return("world")). The 3-character hash is derived from rlang::hash() over trimmed, 80-char-truncated content. The display.markdown shown to users remains a clean code block.
• New btw_tool_files_edit tool: Uses hashline references from the read output to make targeted edits — replace, insert_after, and replace_range actions. Hashes are validated against the current file state, so stale edits are rejected atomically. Multiple edits in a single call are applied bottom-to-top to preserve line numbering.
• include_hashline parameter: Defaults to FALSE in the _impl() function so internal callers like btw_this() get clean output. Only the model-facing tool wrapper passes TRUE.
• Smart edit responses: The edit tool returns updated hashline references for edited regions and surrounding context, so the model can make follow-up edits without re-reading the entire file. Nearby edits (within 10 lines) are merged into a single contiguous region. When edits change the line count, a shift hint tells the model how to adjust cached line numbers (e.g. "update line numbers by +2 (old line 14 → new line 16)").

Edit response format

The response is tiered by complexity:

Tier	Condition	Response
1	Single region, no line count change	Hashlines for region with 1 line of context on each side. No shift hint.
2	Single region, line count changed	Hashlines for region. Shift hint with offset and one concrete old → new example.
3	Multiple disjoint regions	Hashlines per region. Per-region shift hint with cumulative offset.

Example tier 2 response (insert 2 lines after line 2):

Applied 1 edit(s) to helpers.R (now 7 lines, previously 5).

2:bbb|line before edit
3:f1a|inserted line A
4:c3d|inserted line B
5:e5f|line after edit

Content below line 5 was not modified.
Cached hashes are still valid — update line numbers by +2 (old line 3 → new line 5).

Verification

library(btw)

# Read a file — model sees hashline-annotated output
result <- btw_tool_files_read("R/tool-files.R", line_start = 1, line_end = 5)
cat(result@value)
# 1:a3f|#' @include tool-result.R
# 2:b1c|NULL
# 3:d4e|
# 4:f2a|hashline <- function(line) {
# 5:c8b|  substr(rlang::hash(substr(trimws(line), 1, 80)), 1, 3)

# Edit using hash references from the read
btw_tool_files_edit("test.R", edits = list(
  list(action = "replace", line = "4:f2a", content = list("new_hashline <- function(line) {"))
))