docs: address v2 release review findings

[theogravity]

Summary

Follow-up to the v2.0.0 release (#61, #63) and the user-facing import sweep (#65), addressing issues surfaced by a four-reviewer documentation pass against the post-v2 docs.

Critical:

• creating-transports.md: drop the stale // already prefix-applied comment on TransportParams.Messages; the core no longer mutates Messages[0].
• llms.txt / llms-full.txt: add transports/cli to the Renderers catalog and install snippet (shipped at v2.0.0 but was missing from both).
• introduction.md: show the lltest "go.loglayer.dev/transports/testing/v2" import alias in the test-capture snippet.

Important:

• GoDoc cross-references bumped to /v2 in transports/cli/cli.go, transports/otellog/otellog.go, doc.go, examples/pretty-modes/main.go.
• migrating-to-v2.md: reframe lede as "two breaking changes" (the import-path bump is itself breaking); add a paragraph about the core no longer mutating caller-owned input; rewrite the Step 1 shell snippet so the trailing-backslash + comment paste-trap is gone; add a callout for the new testing.LogLine.Prefix field.
• configuration.md / llms.txt: reframe the Prefix inline comment so it no longer claims the core prepends.

Style:

• Em-dash sweep across llms.txt and llms-full.txt (~50 occurrences) per the project's no-em-dashes-anywhere rule.
• llms-full.txt: typo rolled-our-own → roll-our-own; reframe the Testing Transport (lltest) and Capture entries with lltest headings since lltest is an alias convention, not the package name.

transports/central left undocumented intentionally: its package GoDoc is already correct for v2, and the module is not in monorel.toml or the v2.0.0 release.

Test plan

• `go build ./...` and `bash scripts/foreach-module.sh build` pass
• `bash scripts/foreach-module.sh test` passes (pre-push hook)
• `bun run docs:build` produces a clean VitePress site
• `grep '—' docs/src/public/llms.txt docs/src/public/llms-full.txt` returns nothing
• All v1-shaped `go.loglayer.dev/transports/` references in user-facing GoDoc are bumped to `/v2`

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