[FEATURE] export: add --format flag (md/json) and --last N selective export

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

/export exports the full conversation as plain text. This misses a core
developer workflow: saving a specific output (summary, plan, change log)
as a clean .md file — without full conversation noise, and without
spending tokens on a model turn to do it.

Current workarounds all have token/friction costs:

• Copy-paste: manual
• Custom skill using Write tool: triggers a model invocation
• MCP wiki sync: also requires tokens

Proposed Solution

Two zero-token additions to /export (both read the existing JSONL transcript):

1. Format flag
/export --format md [filename] → preserves markdown, saves as .md
/export --format json [filename] → structured JSON with roles + content
/export --format txt [filename] → current default (plain text)

2. Selective export
/export --last 1 [filename] → last assistant response only
/export --last 3 [filename] → last 3 assistant turns

Combined:
/export --format md --last 1 .claude/exports/sprint-plan.md

Why zero-token
The JSONL transcript is already written to
~/.claude/projects/<hash>/<session>.jsonl on every turn. /export
already reads this — it only needs a format conversion step and a
message-slice filter. No model invocation required.

Parity justification
/copy [N] already supports Nth-latest response selection. /export
having the same --last N selectivity is a natural parity gap.

I'd like to contribute a fix
Happy to open a PR. The change should be scoped to the /export
command handler — adding arg parsing and format-aware serialization.

Workaround

I'm contributing a plugin (export-session) to plugins/ as a working
workaround while this native flag is considered.
https://github.com/anthropics/claude-code/pull/54777

Related issues

• #34339 — /export strips markdown (bug; --format md would fix as side effect)
• #26890 — auto-save before compaction (different trigger)

Alternative Solutions

_No response_

Priority

Medium - Would be very helpful

Feature Category

CLI commands and flags

Use Case Example

_No response_

Additional Context

_No response_