Shorten the diff-metadata env var to OSC1717_METADATA

Pager authors reviewing the protocol asked to drop the EMIT_ prefix: the
OSC number already names the protocol, so EMIT_ was redundant. Rename the
handshake variable to OSC1717_METADATA in the spec and in the two places
lazygit advertises it (the pager PTY and the metadata probe).

diff-line-metadata-osc-spec.md

@@ -76,7 +76,7 @@ stays in the pager, which is the only component that has it.
 ## 3. Negotiation handshake
 
 ```
-EMIT_OSC1717_METADATA = V1[,V2,…]
+OSC1717_METADATA = V1[,V2,…]
 ```
 
 - The **host** sets this environment variable on the pager subprocess to the list
@@ -404,7 +404,7 @@ mapping; recorded as a v2 candidate, not taken (§9).
    in the large empty 1400–5000 band (only iTerm2's `1337` is nearby). There is no
    central registry, so this is "verified unused across the terminals that matter,"
    not "allocated." If you know of a terminal that interprets `1717`, please say so.
-2. **The env-var name and grammar** (`EMIT_OSC1717_METADATA=V1,…`).
+2. **The env-var name and grammar** (`OSC1717_METADATA=V1,…`).
 3. **The token-vs-line mismatch** (§8) — should there be an `m` type, or is
    host-side inference the right home for it?
 4. **Can your pager actually produce all four fields per region?** In particular

pkg/commands/git_commands/diff.go

@@ -34,7 +34,7 @@ const metadataHandshake = "\x1b]1717"
 // caches it per pager.
 //
 // No PTY is needed: git needs a terminal to decide to invoke a pager, but the pager
-// itself emits the handshake whenever EMIT_OSC1717_METADATA is set, so we can run it
+// itself emits the handshake whenever OSC1717_METADATA is set, so we can run it
 // directly with empty input.
 //
 // A git-config external diff driver (useExternalDiffGitConfig) is chosen per file via
@@ -75,7 +75,7 @@ func (self *DiffCommands) externalDiffEmitsMetadata(extDiffCmd string) bool {
 }
 
 func (self *DiffCommands) probeEmitsMetadata(cmdObj *oscommands.CmdObj) bool {
-	cmdObj.AddEnvVars("EMIT_OSC1717_METADATA=V1")
+	cmdObj.AddEnvVars("OSC1717_METADATA=V1")
 	// The pager may exit non-zero on the synthetic input; we only care about whether
 	// it emitted the handshake first, and the output is captured either way.
 	output, _ := cmdObj.RunWithOutput()

pkg/gui/pty.go

@@ -105,7 +105,7 @@ func (gui *Gui) newPtyTask(view *gocui.View, cmd *exec.Cmd, prefix string) error
 		// an OSC sequence we can read back (see diff-line-metadata-notes.md). A
 		// pager that doesn't understand it ignores the variable, so this is safe to
 		// set unconditionally.
-		cmd.Env = append(cmd.Env, "EMIT_OSC1717_METADATA=V1")
+		cmd.Env = append(cmd.Env, "OSC1717_METADATA=V1")
 
 		manager := gui.getManager(view)