feat(ui): make file and hunk spacing configurable

[Ajay-Satish-01]

Summary

• Adds file_gap / --file-gap (0–8, default 1) so the space between files in the review stream is configurable. 1 keeps the current ─ rule, 0 hides it, and larger values add blank rows above the rule.
• Adds hunk_gap / --hunk-gap (0–8, default 0) so later hunks can have blank rows before @@ (still inserted when hunk headers are hidden).
• This is presentation-only in the TUI/OpenTUI stream (not shared review geometry, View-menu persistable, static pager, or browser CSS).

Fixes #839

Test plan

• bun run start -- diff --no-sidebar --mode stack --file-gap 3 --hunk-gap 2 on a repo with at least two files and two hunks
• --file-gap 0 hides the ─; --file-gap 1 is the current look
• --hunk-gap 2 puts two blanks before later @@; first file/hunk stay flush
• --no-hunk-headers --hunk-gap 2 still inserts blanks before later hunk content
• Config file_gap / hunk_gap apply; CLI flags override; --file-gap 9 and --hunk-gap abc error
• [ / ] still land on hunk content, not the spacer

Verified: unit tests for parse/config/plan/geometry; PTY layout tests for --file-gap 3 and --hunk-gap 2; live PTY frames for 0/1/3 file gaps and 0/2 hunk gaps (including hidden headers). format:check, lint, typecheck, check:docs, deps:check, and bun run test passed.

Screenshots

Real Hunk TUI (PTY capture), 100×32, --no-sidebar --mode stack, default theme, macOS.

Default look (file_gap=1, hunk_gap=0)

One ─ rule between files; no extra space before later hunks.

--file-gap 0

Hides the ─; the next file header sits on the previous body.

--file-gap 3

Two blank rows, then the ─ rule, then beta.ts.

--hunk-gap 2

Two blank rows before the later @@ in alpha.ts. First hunk stays flush.

--file-gap 3 --hunk-gap 2

Both gaps together.

--no-hunk-headers --hunk-gap 2

Blanks still appear before later hunk content when hunk headers are hidden.

[greptile-apps]

PR author is not in the allowed authors list.

[vercel]

@Ajay-Satish-01 is attempting to deploy a commit to the Modem Team on Vercel.

A member of the Team first needs to authorize it.

[Ajay-Satish-01]

@benvinegar, would you be able to take a look at this? Thanks

[benvinegar]

I'm good with the behavior (adding the config + feature) - good call

I just want to look over the code first